You would solve a problem like this:
3(2x-5)=21
6x-15=21... I multiplied 2x and -5 by 3
6x-15=21
6x-15+15=21+15... I added 15 to both sides so 6x can be alone
6x=36
6x/6=36/6... I divided by 6 from both sides to leave x alone to get the answer:
x=6.
