In this case you expand the brackets all the brackets (noting that -(4x+7) is

-4x - 7):

3y - 4x - 7 -y+11+ 2x - y + 12

collect together all the terms with the same unknown:

(3y - y - y) + (-4x + 2x) - 7 +11 +12

tot up how many of each variable you have:

(1y) + (-2x) + 16

remove brackets:

y - 2x +16

RonL