Question

How do I find the value of X

Answer 1

You could either:
a) Use the Pythagorean Theorem
b) Use Sine/Cosine/Tangent Inverse to find one of the angles, and then use that angle to find x.

Answer 2

X^2 = 20^2 - 8^2
x^2 = 400 - 64
x^2 = 336
x = 18.3

hope it helps