8)a)x²y+5xy²-3x²y+6xy²-2xy²+3x²y=x²y+9xy²;
b)(5a²+3ab)+(b²-2ab)-(a²+3b²-4ab)+(a²-b²)=5a²+3ab+b²-2ab-a²-3b²+4ab+a²-b²=3a²+5ab-3b²
c)(6m+3n)-(2n+3m+5)-(3m-4n-6)=6m+3n-2n-3m-5-3m+4n+6=5n+1;;
d)(13m²+2mn-n²)-(2m²+3mn+4n²-(10m²+2mn)=13m²+2mn-n²-2m²-3mn-4n²-10m²-2mn=m²-3mn-5n²
