Plot a linear equation

The graph of $ax + by = c$ is a straight line. You will find where it meets the axes, work out $y$ for any $x$, and list five points. Then the program draws the line through them.

The idea. Put $y = 0$ for the x-intercept, $\dfrac{c}{a}$, and $x = 0$ for the y-intercept, $\dfrac{c}{b}$. For any $x$, $y = \dfrac{c - ax}{b}$.

Your tasks.

1. In intercepts, return the pair (c / a, c / b).
2. In y_on_line, return the $y$ that goes with x.
3. Fill points with the pairs (x, y) for x from $0$ to $4$, for the line $2x + 5y = 20$.