so from the interior we know that we have 2 angles
1 is 63 degrees and 1 is 90 degrees because we see that there is a right angle and all right angles = 90 degrees, we also know that the sum of all angles in a triangle should equal 180 degrees. if we have a 63 degree angle and a 90 degree angle we know that the sum of both of them is 153 degrees we still need to find that missing angle and we can do that by minusing 153 from 180 which is 27 so the measurement of the interior angle of the triangle is 27 but that is not what we are looking for but the answer is already given to us because we know that a straight line is 180 degrees so we just minus that missing angle from 180 degrees we conclude that A = 153 degrees
