You would calculate this answer with the Pythagorean theorem. The formula is a² + b² = c², which basically means that the sum of two sides squared would equal the side that is missing.
Currently, we are given 6.5 for side 1. The angle measurments we are given is 45º and 90º [the right angle], which means that if we subtract the sum of 45 and 90 from 180 [the total degree amount of every triangle] we can get the missing angle. With the missing angle, we can calculate the length of the second side.
45º + 90º = 135º. 180º - 135º = 45º.
Now that we have the angles of all three sides [45º, 90º, 45º] we can find the second length of the triangle. The fact that two angles are the same [45º] means that the length is also the same.
Thus proving that M is also 6.5 in length, we can square 6.5 and add both sides to each other once squared.
