A^2+2ab+3b^2-3a^2-4ab+b^2+2a^2-3ab+4b^2
А²+2ab+3b²-3a²-4ab+b²+2a²-3ab+4b²=(a²-3a²+2a²)+(2ab-4ab-3ab)+(3b²+b²+4b²)=-5ab+8b²