1-. 5x-10y
2-.3x^6y+6x^5y
3-.x^2+5x-3x-15=x^2+2x-15
4.-x^2-4x+7x-28=x^2+3x-28
5-.x^2-3x-4x-12=x^2-7x-12
6-.-18x^2-24x-6x+8=-18x^2-30+8
7-. -8x^2+12x
8-. -18x^2y^3+20y^2
9-.12x^4+8x^3-4x^2+14x+18x^3+12x^2-6x+21=12x^4+26x^3+8x^2+8x+21