First you would identify the sides given. We have 3cm, which is one of the legs, or bases, and 5cm which is the hypotenuse because it is the longest side.
To find X, we would start at the angle we need to find and see that the given values are opposite and hypotenuse. Using SOH CAH TOA we know that we will use sin because sin=opposite divided by hypotenuse
The final calculation is
sin^-1=3/5
Angle X=36.8 degrees
(sin^-1 is just the inverse of sine, which can usually be found on a scientific calculator)