Use Pythagoras twice.
You have AB and BC, so use Pythagoras to write down an expression for AC.
Now use Pythagoras again in the triangle ACF.
Solve the resulting equation for x.
Guest is correct ...
These steps can be reduced into one step by realizing that they produce a "three dimensional" Pythagorean Theorem.
In two dimensions, c2 = a2 + b2,
in three dimensions, d2 = a2 + b2 + c2, where a, b, and c represent length, width, and height.