(6x + 9y) + (7x - 4y)
Remove parenthesis:
6x + 9y + 7x - 4y
Add like terms:
(6x + 7x) + (9y - 4y)
(13x) + (5y)
13x + 5y
(6x + 9y) - (7x - 4y)
Remove parenthesis (keep in mind law of signs)
6x + 9y - 7x + 4y
Add like terms:
(6x - 7x) + (9y + 4y)
(-x) + (13y)
-x + 13y
or
13y - x