How to find the missing angle of a right triangle given two sides?

By using trigonometry function
sin , cos , tan...

    |\
    | \
    |  \
    |   \
    |    \
    |     \H
    |      \
O  |       \
    |        \
    |x\
        A
O is opposite the angle
A is adjacent to the angle
H is the hypotenuse (longest side)
sin(x) = O/H
cos(x) = A/H
tan(x) = O/A
(you can remember using SohCahToa
Substitute the sides in and you will find the missing angle.
If H was 30 and A was 15:
cos(x) = 15 / 30 = 0.5
cos(x) = 0.5
Then you would do inverse sin (sin^-1) to find x:
cos^-1(0.5) = 60
x = 60°