The Pythagorean Theorem states that a^2 + b^2 = c^2.
In this case, 'a' would be 'x' and 'b' would be 'y', leaving 'c' to be 'z'.
x^2 + y^2 = z^2
x^2 + (2.5)^2 = (6.5)^2
x^2 + 6.25 = 42.25
Subtract 6.25 from both sides
x^2 = 36
Take the square root of both sides
x = 6 cm