Question 384035
Just multiply:

{{{(x + 2y)(x^2 - 2xy + 9y^2) = x^3 - 2x^2y + 9xy^2 + 2x^2y - 4xy^2 + 18y^3}}}

We can cancel out {{{2x^2y}}} to obtain

{{{x^3 + 5xy^2 + 18y^3}}} (in^2)