#### Answer

$[-3, 6]$
Refer to the image below for the graph.

#### Work Step by Step

Factor the trinomial to obtain:
$(x-6)(x+3)\le0$
Next, find the zeros of each factor.
To find value/s of $x$ that will make each factor equal to zero, equate each factor to zero then solve each equation:
$\begin{array}{ccc}
&x-6=0 &\text{ or } &x+3=0
\\&x=6 &\text{ or } &x=-3
\end{array}$
Next step is to find the intervals.
The zeros $-3$ and $6$ divide the number line into three intervals, namely:
$(-\infty, -3), (-3, 6), \text{ and } (6, +\infty)$.
$\bf\text{Make a table of signs}.$
(refer to the attached image below)
$\bf\text{Solve}$
From the table of signs, it can be seen that $(x-6)(x+3)\le 0$, which is equivalent to $x^2-3x-18\le0$, in the interval $(-3, 6)$.
The inequality involves $\le$ therefore the endpoints $-3$ and $6$ are part of the solution set.
Thus, the solution set is $[-3, 6]$.
To graph this, plot solid dots at $-3$ and $6$ then shade the region between them.
(refer to the attached image in the answer part above)