diff --git a/2023/day09/part2.ts b/2023/day09/part2.ts
index 3692f70..8b51fa6 100644
--- a/2023/day09/part2.ts
+++ b/2023/day09/part2.ts
@@ -1,6 +1,32 @@
 export function answer(input: string): number {
-  console.log(input);
-  return 42;
+  const seqs = input
+    .split("\n")
+    .map((l) => l.split(" ").map((x) => parseInt(x)));
+
+  return seqs.reduce((sum, seq) => (sum += prevNum(seq)), 0);
+}
+
+function prevNum(seq: number[]): number {
+  const seqs = [seq];
+  const reduce = (s: number[]): number[] => {
+    let [prev, ...rest] = s;
+    return rest.map((x) => {
+      const next = x - prev;
+      prev = x;
+      return next;
+    });
+  };
+
+  while (!seqs[0].every((x) => x === 0)) {
+    // console.log(reduce(seqs[0]));
+    seqs.unshift(reduce(seqs[0]));
+  }
+  for (const [i, s] of seqs.entries()) {
+    if (i === 0) continue;
+    const prevSeq = seqs[i - 1];
+    s.unshift(s[0] - prevSeq[0]);
+  }
+  return seq[0];
 }
 
 if (import.meta.main) {
diff --git a/2023/day09/puzzle.md b/2023/day09/puzzle.md
index d243946..62d5a42 100644
--- a/2023/day09/puzzle.md
+++ b/2023/day09/puzzle.md
@@ -97,8 +97,36 @@ If you find the next value for each history in this example and add them togethe
 
 Analyze your OASIS report and extrapolate the next value for each history. *What is the sum of these extrapolated values?*
 
-To begin, [get your puzzle input](9/input).
+Your puzzle answer was `1641934234`.
+
+The first half of this puzzle is complete! It provides one gold star: \*
+
+\--- Part Two ---
+----------
+
+Of course, it would be nice to have *even more history* included in your report. Surely it's safe to just *extrapolate backwards* as well, right?
+
+For each history, repeat the process of finding differences until the sequence of differences is entirely zero. Then, rather than adding a zero to the end and filling in the next values of each previous sequence, you should instead add a zero to the *beginning* of your sequence of zeroes, then fill in new *first* values for each previous sequence.
+
+In particular, here is what the third example history looks like when extrapolating back in time:
+
+```
+5  10  13  16  21  30  45
+  5   3   3   5   9  15
+   -2   0   2   4   6
+      2   2   2   2
+        0   0   0
+
+```
+
+Adding the new values on the left side of each sequence from bottom to top eventually reveals the new left-most history value: `*5*`.
+
+Doing this for the remaining example data above results in previous values of `*-3*` for the first history and `*0*` for the second history. Adding all three new values together produces `*2*`.
+
+Analyze your OASIS report again, this time extrapolating the *previous* value for each history. *What is the sum of these extrapolated values?*
 
 Answer:
 
-You can also [Shareon [Twitter](https://twitter.com/intent/tweet?text=%22Mirage+Maintenance%22+%2D+Day+9+%2D+Advent+of+Code+2023&url=https%3A%2F%2Fadventofcode%2Ecom%2F2023%2Fday%2F9&related=ericwastl&hashtags=AdventOfCode) [Mastodon](javascript:void(0);)] this puzzle.
\ No newline at end of file
+Although it hasn't changed, you can still [get your puzzle input](9/input).
+
+You can also [Shareon [Twitter](https://twitter.com/intent/tweet?text=I%27ve+completed+Part+One+of+%22Mirage+Maintenance%22+%2D+Day+9+%2D+Advent+of+Code+2023&url=https%3A%2F%2Fadventofcode%2Ecom%2F2023%2Fday%2F9&related=ericwastl&hashtags=AdventOfCode) [Mastodon](javascript:void(0);)] this puzzle.
\ No newline at end of file
diff --git a/2023/day09/test.ts b/2023/day09/test.ts
index 94b4ef2..279db5c 100644
--- a/2023/day09/test.ts
+++ b/2023/day09/test.ts
@@ -11,7 +11,11 @@ Deno.test("part1", () => {
   assertEquals(p1.answer(examples), 114);
 });
 
-// Deno.test("part2", () => {
-//   const examples = ["abc", "def"].join("\n");
-//   assertEquals(p2.answer(examples), 42);
-// });
+Deno.test("part2", () => {
+  const examples = [
+    "0 3 6 9 12 15",
+    "1 3 6 10 15 21",
+    "10 13 16 21 30 45",
+  ].join("\n");
+  assertEquals(p2.answer(examples), 2);
+});