The vertical angle diagonally across the equation, (3x +9) is also (3x+9).
Since m is perpendicular to n, that means that (3x + 9) + (x + 21) = 90° This is so because perpendicular lines form right angles which have the measure of 90°
3x + 9 + x + 21 = 90
4x + 30 = 90
4x = 60
x = 15
