Answer
$x^3+7x^2+21x+27$
Work Step by Step
Using the Distributive Property and then combining similar terms, the given expression, $
(x+3)(x^2+4x+9)
,$ is equivalent to
\begin{array}{l}\require{cancel}
x(x^2)+x(4x)+x(9)+3(x^2)+3(4x)+3(9)
\\\\=
x^3+4x^2+9x+3x^2+12x+27
\\\\=
x^3+(4x^2+3x^2)+(9x+12x)+27
\\\\=
x^3+7x^2+21x+27
.\end{array}