Kenneth S. answered 01/11/17
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Expert Help in Algebra/Trig/(Pre)calculus to Guarantee Success in 2018
If the hypotenuse is y in an isosceles right triangle, then each leg measures y/√2.
Now you have y/√2 = 13/√x, using the latter expression as the given side length.
Manipulate this by various algebraic operations to solve for x (in terms of y squared)!