Let's start with what we know:

And we are trying to put it into this form:

Here is what it would look like:

You can check you work by using the FOIL method (first, outer, inner, last)
Subtract 6 from both sides, leaving only 3x on one side and 15 on the other. divide 15 by 3 and get 5 for x. 3(5)+6=21
The answer is the last one !
Answer:
12
Step-by-step explanation: