If you're attempting to find a missing side length for a ride triangle, take into though that one of the angles is 90 degrees. If your other angle is (for example) 30 degrees, then add your two angles. 90 + 30 = 120.
We know that all triangles equal 180 degrees.
Take 120 from 180 to get 60.
Your missing side length is 60 degrees.
I hope this helps! Message me if you need further help.