Need help with this problem

Since the shape is a square, it has 90° angles.

if cut in half, it creates a 45-45-90 triangle.
using this, you can use the ratio of 1:sqrt2 to find x. so the ratio would be x:6 to 1:sqrt2.

divide 6 by sqrt2 to find x, and then rationalize for it to become 6sqrt2/2, which would simplify even further to 3sqrt2

When doing these problems remember to distribute parentheses and combine like terms
hope this helps!