Since this is a right triangle, we can use trigonometry to solve. We'll use one of the three main trigonometric functions: sine, cosine, or tangent.
Remember: SOH-CAH-TOA
Looking at our unknown angle, we know the adjacent side and the hypotenuse. Therefore, we should use cosine.
cos(x) = 17/20
To solve, use the inverse cosine function (cos^-1) on your calculator.
x = 31.8 degrees
Hope this helps!