Answer
$a.$ and $b.$ See image below.
c $\quad f^{-1}(x)=\displaystyle \frac{x+6}{3}$
Work Step by Step
$a. $Set up a table for several x and their function values ... for points on the graph.
Joint the points. (Red graph on the image)
b. Using the table in (a), plot (y,x) for each (x,y).
(The points (y,x) are reflections about the line $y=x$)
Joint the points. (Green graph on the image)
c.
$\left[\begin{array}{ll}
\text{In the equation, } & \\
\text{ replace f(x) with y} & y=3x-6\\
\text{swap x and y} & x=3y-6\\
\text{solve for y} & x+6=3y\\
& \dfrac{x+6}{3}=y\\
\text{replace y} & \\
\text{with }f^{-1}(x) & f^{-1}(x)=\dfrac{x+6}{3}
\end{array}\right]$