trying to figure the length of a right triangle when the other two sides are 17 and 15?

Do you mean that the sides adjacent to the right angle are 17 and 15?

yes,

a^2 + b^2 = c^2

17^2 + 15^2 = c^2
289 + 225 = c^2
514 = c^2
22.67 = c

this is how you find the answer to a right triangle for the length with one side being 17 and the other 15?

Right. The hypotenuse is 22.67

i think i forgot to tell you there is alittle square box in the bottom of the right triangle when it is asking for the missing length. I am sorry

Is the square box in the angle adjacent to the two sides -- 15 and 17?

if you look at a right triangle there is 17 at the top and 15 at the bottom with the little square in the left bottom to where you have to find the length and the 15 this would be easier if i could draw it for you.

Do you have to find the longest side?

Please study this site.

http://www.mathsisfun.com/pythagoras.html

Are 15 and 17 in the same places on your triangle as a and b are on the diagram in the website?

you are finding what(a)would be with b being 17 and c being 15