Answer
$x^3+9x^2y+27xy^2+27y^3$
Work Step by Step
Using $(a+b)^3=a^3+3a^2b+3ab^2+b^3$ or the cube of a binomial, the given expression, $
(x+3y)^3
,$ is equivalent to
\begin{array}{l}\require{cancel}
(x)^3+3(x)^2(3y)+3(x)(3y)^2+(3y)^3
\\\\=
(x)^3+3(x^2)(3y)+3(x)(9y^2)+(27y^3)
\\\\=
x^3+9x^2y+27xy^2+27y^3
.\end{array}
