-[3/4a+2b]-[1/2a-b-2]=

-3/4a - 2b - 1/2a + b + 2 = -3/4a - b - 2/4a + 2 = -5/4a - b + 2

-[3/4a+2b]-[1/2a-b-2]= -3/4a-2b-1/2a+b+2= -5/4a-b+2 :)