On way to solve this is to write:
, and simplify plug it into your calculator.
Another way is to consider a triangle. If you take the sin of an angle in a triangle, it gives the ratio of the length of the side opposite that angle and the hypotenuse.
Hence the hypotenuse of our triangle is 5, and the opposite side from angle x is 1. From pythagorus we calculate that the side adjacent to the angle x is
Now you know all lengths, you can work out the angle using the cosine rule.