Hello!
Answer:

*The answer must have a POSITIVE sign!*
Explanation:
First, you subtract by 12 from both sides of an equation.

Simplify by equation.

Then, you add by 9 from both sides of an equation.

Add by the numbers from left to right.

, which is our final answer.
Hope this helps!
Thanks!
-Charlie
Have a great day! :)
:D