First combine the like terms together first.
1) -15 = 3a + 18
Now you have to get the variable on one side.
2) Do -15 - 18, which is -33. The problem now becomes -33 = 3a.
3) Now you have to solve for a. The next step is -33 divided by 3, which becomes -11 = a.
4) - 11 is now your answer for a.
5) If you want to, you can check.
-15 = 5 (-11) + 12 - 2(-11) + 6
-15 = -55 + 12 + 22 + 6
-15 = -15
- 15 =
