Answer
Please see image
range = $\{0,3,6,9,12,15\}$
Work Step by Step
$\left[\begin{array}{lll}
x & -3x+9 & (x,y)\\
\hline-2 & 6+9=15 & (-2,15)\\
-1 & 3+9=12 & (1,12)\\
0 & 0+9=9 & (0,9)\\
1 & -3+9=6 & (1,6)\\
2 & -6+9=3 & (2,3)\\
3 & -9+9=0 & (3,9)
\end{array}\right]$
The range (second column values) is
$\{0,3,6,9,12,15\}$