Explanation:
x is the height of the shape towards the 18 side.
this is also the second leg of the right-angled triangle with 16 as its Hypotenuse.
that makes the first leg 18-5 = 13.
so, Pythagoras gives us this second leg and therefore x :
16² = 13² + x²
256 = 169 + x²
87 = x²
x = sqrt(87) = 9.327379053... ≈ 9.3