[yorwba]

The other replies are already telling you that these are just examples. I want to stress that these are completely unrelated examples, which is bad form IMO.

If the first example had been kept, then the second would have been "Error = (6 - (2·3 + 1))² + (9 - (2·6 + 1))² + (18 - (2·12 + 1))² = (-1)² + (-4)² + (-7)² = 66", which is what compute_error_for_line_given_points evaluates to.

The third would have been "Error = (6 - (m·3 + b))² + (9 - (m·6 + b))² + (18 - (m·12 + b))² = 3·b² + 42·b·m - 66·b + 189·m² - 576·m + 441" and its derivative would have to be taken in two directions, giving "dError/dm = 42·b + 378·m - 576" and "dError/db = 6·b + 42·m - 66". Visualizing that slope would require a 3D plot.