PEMDAS. First, use the distributive property with -3(2x) + -3(-4) to get -6x + 12. Remember the 9, so you get 9 - 6x + 12. Now add 9 and 12 to get 21 - 6x.
The answer is 21 - 6x
You first do distributive property and multiply 3 by what's in the parenthesis
3 * 2x = 6x + 3 * -4 = 12
Then 9 - 6x + 12
Which gives you 21 - 6x