Using Pythagorean's Theorem of a^2+b^2=c^2, with a and b being two legs and c being the hypotenuse.
You can find that the leg(15) squared is 225. The hypotenuse(17) squared is 289.
289-225=64
The square root of 64 is 8
So the missing leg of the triangle has a length of 8 units.
