Since x=x, this is an isosceles right triangle.  By the Pythagorean Theorem:
h^2=a^2+b^2  (the hypotenuse squared is equal to the sum of the squared sides)
5^2=x^2+x^2
25=2x^2
2x^2=25
x^2=25/2
x=√(25/2)
x=5/√2  now if we rationalize the denominator...
x=(5√2)/(√2√2)
x=(5√2)/2