Use the pythagoren theorem to solve x

Well the equation is (A^2)+(B^2)=(C^2)

A= x
B= 2x
C will always be the longest side because it is the Hypotenuse = 25

So if you plug in those numbers into the equation...

(x^2) + (2x^2) = (25^2)

x^2 + 4x^2 = 625

Combine like terms

5x^2 = 625

Divide by five to both sides

x^2 = 125

Then Square root,

x = sqrt(125)

x = sqrt(25* 5)

x = 5sqrt(5)