How do you find a angel when you know the sides?

Lets say its a right angle : we know that it is 90 degrees

Since triangles are congruent to each triangle, each triangle has half as many degrees, namely 180. So this is true for any right triangle.

use The Law of Cosines first to calculate one of the angles

then use The Law of Cosines again to find another angle

and finally use angles of a triangle add to 180° to find the last angle.

cos (C) = (a2 + b2 - c2) / 2ab

cos (A) = (b2 + c2 - a2) / 2bc

cos (B) = (c2 + a2 - b2) / 2ca