Question 180778
{{{x^3y^2+2x^2y^3+3xy-4+x^2y^3-7x^3y^2+2xy+1}}}
In order to add, we just find like terms.  So, for example, {{{x^3y^2}}} and {{{-7x^3y^2}}} are alike because the x's are cubed and the y's are both squared.  So, we get that 
{{{x^3y^2+2x^2y^3+3xy-4+x^2y^3-7x^3y^2+2xy+1 = -6x^3y^2+3x^2y^3+5xy-3}}}


{{{(x^2+2y^2-xy)(x^2+7xy+y^2)}}}
To multiply, we need to distribute each term of the first trinomial through to each term of the second trinomial.  So,
{{{(x^2+2y^2-xy)(x^2+7xy+y^2) = (x^2)(x^2)+(x^2)(7xy)+(x^2)(y^2)+(2y^2)(x^2)+(2y^2)(7xy)+(2y^2)(y^2)-xy(x^2)-xy(7xy)-xy(y^2)}}}
{{{x^4+7x^3y+x^2y^2+2x^2y^2+14xy^3+2y^4-x^3y-7x^2y^2-xy^3}}}
{{{x^4+6x^3y-4x^2y^2+13xy^3+2y^4}}}