Answer:
1) (7x - 8) - (2x + 4)
= 7x - 8 - 2x - 4
= 5x - 12
2) (8x - 6y) - (2x - y)
= 8x - 2x -6y + y
= 6x - 5y
3) (12xy + 5y) - (23xy - 3y)
= 12xy - 23xy + 5y + 8y
= -11xy + 13y
= 13y - 11xy
