By using Pythagoreans Theorem: a^2 + b^2 = c^2

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