(1)x-y+5x-4y
=x+5x-y-4y
=6x-5y
(2)2/3a-1/2ab+3/4a+ab-b
=2/3a+3/4a-1/2ab+ab-b
=17/12a+1/2ab-b
(3)4(x-y)+2(x-y)-7(x-y)-3(x-y)
=(4+2-7-3)(x-y)
=-4(x-y)
=-4x+4y