Plot the following two graphs on the same cartesian plain

f(x) = 3x -2
f^ -1(x) = x/3 +2/3

The two functions are 1st degree polynomial, so they are both lines.

In order to draw a line, you have to sample two points and connect them.

To sample points from an equation, choose any value for x and compute the correspondent value for y.

For example, if we choose x=0 and x=1 for the first equation, we have

So, the line passes through the points and

Similarly, if we choose x=0 and x=3 for the first equation, we have

So, this line passes through the points and

If you draw the two points for each line and connect the pairs, you'll have the two lines.