Answer:
x = 15°
Explanation:
If you use Vertical Angles, you can move the two algebraic expressions next to each other and totaling 90° (because m and n are perpendicular)
x + 21 + 3x + 9 = 90°
combine like terms.
4x + 30 = 90
subtract 30 from both sides.
4x = 60
divide by 4.
x = 60/4
x = 15