m+n=(5a²-3ab+2b²)+(4a²-3ab)=5a²-3ab+2b²+4a²-3ab=9a²-6ab+2b²
m-n=(5a²-3ab+2b²)-(4a²-3ab)=5a²-3ab+2b²-4a²+3ab=a²+2b²