Answer:
66/29
Explanation:
-9x - 21 = 20x - 87
Add 87 to both sides to cancel out the -87 on the right side:
-9x + 66 = 20x
Add 9x to both sides to cancel out the -9x on the left side:
66 = 29x
Divide both sides by 29 to isolate the variable:
66/29 or 2.275862....