A toolkit for straight lines

You will write three functions for lines: a test for collinear points, the line through two points, and the intercepts of a line. The checker tests them on many cases, including vertical lines.

The idea. Points $A$, $B$, $C$ are collinear when the slopes of $AB$ and $AC$ match. Cross-multiplying avoids a division by zero. A line is $y = mx + c$ with $c = y_1 - m x_1$, and its intercepts are $-\dfrac{c}{m}$ and $c$.

Your tasks.

1. Finish collinear(a, b, c). Return True or False, using cross-multiplication and no division.
2. Finish line_through(p, q): return the pair (m, c), or None for a vertical line.
3. Finish intercepts(m, c): return (x_intercept, y_intercept). The $x$-intercept is None when m is $0$.