13x+3y-7z - (2x+3y+5z)
4x+3y-8z-7x+8y-5z
8x+3y+2z +3x-4y+1z
8x2-2x+1-3x2-5x+8
7a-3n+12g + 2a-4n-15g
-3x+11y-13z
11x-1y+3z
11x-12z
9a-7n-27g
5x2-7x+9
