Well we know that the angles in a triangle have to add up to 180 degrees so if you call this angle theta, if you call that angle theta we know that theta plus 45 degrees plus 90 degrees must add up to 180 degrees or theta plus 135 must add up to 180 so if you subtract 135 from both sides you get that theta is equal to 45 degrees so this is also equal to 45 degrees. 