When finding a missing side for a right triangle you want to use the Pythagorean theorem. The formula of this equation is
a^2+b^2=c^2
Now you just need to plug in the numbers
a=15
b=x
c=17. *(note) the hypotenuse will always be c
the equation should look like this....
15^2+x^2=17^2
225+x^2=289. solve for x
-225. -225
x^2=64
√x=√64.
(To simplify something that is squared you
need to take the square root.)
So the final answer should be..
x=8