Answer:
x = 15
Step-by-step explanation:
You can use the opposite angles property to solve this problem. By using the property, the opposite angle of x+21 must also be x+21. So therefore, you can setup the equation x+21+3x+9=90. Simplify and you should get 4x+30=90. Subtract 30 on both sides and you get 4x=60. Divide 4 on both sides and you get x=15.
