+505 
Notice that you can form a square in the left corner with side lengths of 5, as shown below. Lines that are tangent to a circle and meet at a point are congruent, so we can label the triangle as shown above. Then, it's just the Pythagorean theorem:
\(18^2+(5+x)^2=(13+x)^2\)
I think you can take it from here.
