you gather the like terms together.
if you have -3y +2x +6w -4 + 3y -5x +2w
you put the like terms together
so for the term y:
you have -3y +3y = 0
for x:
2x - 5x = -2x
for w:
6w + 2w = +8w
and you also had -4. if you put them together you get
0 -2x +8w -4
which is
-2x +8w -4
hope this helps