Basic trigonomic ratios (θ is the reference angle):
Sine θ = opposite / hypotenuse
Cosine θ = adjacent / hypotenuse
Tangent θ = opposite / adjacent
Given the diagram, we can see that the reference angle is x. We are also given the hypotenuse (9) and the opposite side length (4). With the opposite and the hypotenuse, we know that we need to use the sine ratio:
sin(x) = 4/9
To solve for an angle given the side lengths, we use the inverse functions: