several ways like somebody said below you can use use pythag assuming u know the other 2 side lengths w/ a^2+b^2=c^2
however if you do not know the other two side lengths you must use trigonometry and sohcahtoa
sin(angle)= the side length opposite the angle to be referred to as O
divided by the longest side length (hypotenuse)or the angle directly across from the 90 degree angle to be referred to as H
cos(angle) the side length adjacent (next to) the known angle to be referred to as A/H
and finally tan(angle)O/A
if you are still having trouble with this id suggest youtubing "get triggy with it" decent video that explains trig pretty well as well as how to find a missing angle rather than a side length
hope this helps -j