Question 843628
Start with the Pythagorean theorem.  A^2 + B^2 = C^2

The hypoteneus is the longest side, or C. 

You also know that one side, A, is 17 feet.
Because the triangle is isoceles you know that legs A and B are the same.
So plug 17 in for both A and B, then solve for C.

17^2+17^=C^2
289+289=C^2
578=C^2
C=the square root of 578, or: 21.04163