#### Answer

$x^{2}+9x+18$

#### Work Step by Step

The length of Roberto's plot is $x+6$ ft.
The width of Roberto's plot is $x+3$ ft.
The area of Roberto's plot is $(x+6)(x+3)$
$T$able of products$:$
$\begin{array}{rlll}
& x & +3 & \\
\hline x |& x^{2} & +3x & \\
+6| & 6x & -18 &
\end{array}$
$=x^{2}+3x+6x+18$
...gather like terms.
=$x^{2}+9x+18$