Question 52700
The trick to this is identifying your like terms.  They have to have the exact same variables raised to the exact same powers.
{{{(2xy^2+2x^3-x^2y)}}}+{{{(-2x^2y+2xy^2-y^3)}}}
{{{(2+2)xy^2+2x^3+(-1-2)x^2y-y^3}}}
{{{4xy^2+2x^3-3x^2y-y^3}}}
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{{{(2xy^2+2x^3-x^2y)}}}-{{{(-2x^2y+2xy^2-y^3)}}}
{{{(2xy^2+2x^3-x^2y)}}}+{{{(2x^2y-2xy^2+y^3)}}}
{{{(2-2)xy^2+2x^3+(-1+2)x^2y+y^3}}}
{{{0xy^2+2x^3+x^2y+y^3}}}
{{{2x^3+x^2y+y^3}}}