Hello there!
Start by removing the parenthesis combining like terms.
(4x + 12) - (6x - 18)
4x + 12 - 6x - 18
4x - 6x + 12 - 18
Next, simplify.
-2x + 12 - 18
-2x - 6
This is the final answer and it cannot be simplified any further, I hope this was helpful!