Take the three and distribute it to every number in the parentheses, you should have 15x + 27 - 12x, now you want to bring the -12x on the other side to make things easier and now you should have -12x + 15x + 27, quickly add the -12x and the 15x to get 3x, now you should have 3x + 27, you can’t really manipulate this in any other way shape or form so this should be your simplification. I hope this was of use to you ^_^