Now,I would get rid of the parenthesis,also remember a double negative is a positive.
6x+ 6y-x+7y
Now add or subtract like terms.
5x+13y
You can check this by put x as 1 and y as 2, and putting it in to both equation. (I do this, you don't have to)
3(2 +4) - (1- 14)
18+ 13= 31
5 + 26= 31
So it would be 5x+13y.
