Hello!
In order to find the answer to this equation, we must isolate x:
6x + 9 - 16x = -21
9 - 16x = -21 - 6x
9 = -21 - 6x + 16x
9 + 21 = -6x + 16x
30 = 10x
To find the final answer to x, we must divide 30 by 10.
x = 3
I hope this helps you! Have a great day!
-MMMski