2) 7x3x+3y-2x3x-3y;
4) 3x4x+4y-6x8x+8y;

2) (7x/3(x+y))-2x/(3(x-y)=
(7x(x-y)-2x(x+y))/3(x-y)(x+y)
=(7x^2-7xy-2x^2-2xy)/
3(x-y)(x+y)=(5x^2-9xy)/
3(x^2-y^2)=(x(5x-9y)/3(x^2-y^2)

4) (6x-6x)/(8(x+y)=0