slope-intercept form

Slope-intercept form is the most “readable” way to write a line: y = mx + b. It is like a recipe card that tells you, in one glance, how steep the line is and where it starts.

Here m is the slope (the steepness) and b is the y-intercept’s height (the value of y when x = 0). To graph it, plot the point (0, b), then use the slope as rise over run to step to a second point and draw the line through them.

It is wonderfully convenient but cannot describe a vertical line, because a vertical line has undefined slope — there is no m to put in. Vertical lines are written separately as x = a.