Since this is a right triangle we can use the Pythagorean Theorem:
h^2=x^2+y^2, and since x and y are both opposite 45° angles, they are equal to one another, so we have:
h^2=2x^2 since h=3
2x^2=9
x^2=9/2
x=√(9/2)
x=3√(1/2) if we rationalize the denominator by multiplying the fraction by √2/√2 we get
x=(3√2)/2