If you have a Right Triangle and you have one angle, alpha. How do you find the other angle, beta?
The interior angles of triangles always add up to a specific number of degrees, and that number is the same for all triangles. Do you know what it is?
Given a right triangle, you should automatically know the number of degrees of the angle that defines it as a right triangle. That is: all right angles are the same specific number of degrees, and a right triangle is only a right triangle because it contains one angle with this specific number of degrees. How many degrees are there in a right angle?
In this problem, you have been given another angle as well. You just add that to the number of degrees there always are in a right angle, and subtract them both from the well known number that all triangles add up to. The difference is the measure of the third angle.