I'm not sure how to find "s". I've tried using a^2+b^2=c^2 but that doesn't match any of the answer choices. If you could show work that'd be great!

The line segments labeled 8, 9, and [tex]s[/tex] form a right triangle, so Pythagoras' theorem holds:
[tex]8^2+9^2=s^2\implies s=\sqrt{8^2+9^2}=\sqrt{64+81}=\sqrt{145}[/tex]
None of the provided choices are correct...