(5х-4у)+(7х+у)= (5х+4у)-(7х-у)= 13х2 - (3-5х+х2)=
1) 5x-4y+7x+y=12x-3y; 2) 5x+4y-7x+y=5y-2x; 3) 13x2-3+5x-x2=12x2+5x-3