To solve the equation, we need to isolate the variable x.

Starting with the given equation:

a(-3)x + 12 = 21

First, we can simplify the equation by multiplying a and -3:

-3ax + 12 = 21

Next, we can move 12 to the other side of the equation by subtracting 12 from both sides:

-3ax = 21 - 12

Simplifying further:

-3ax = 9

To isolate x, we divide both sides of the equation by -3a:

x = 9 / (-3a)

Therefore, the solution to the equation is x = 9 / (-3a).