remember you can add like terms exg, 3y+4y=7y 3y^2+4y^2=7y^2, but cannot add 3y^2 and 4y, or 3 and 4y so
3a+2w-5a-9w
group like trems
3a-5a+2w-9w
subtract
-2a-7w
3y^2+2w^2-y^2-w^2
group like terms (think of w^2 and y^2 as 1w^2 and 1y^2)
3y^2-y^2+2w^2-w^2
subtract
2y^2+w^2
2x+2x=4x
