Slope and the Algebra of Straight Lines

A straight line is written y = b + mx. The b term is where the line meets the vertical axis, the value of y when x is zero. The m term is the slope, the rise in y for every one-step move to the right. In the book's example b is 9 and m is 3, so the line starts at 9 and climbs by 3 each time x goes up by one.

The dots are the table's values, plotted and joined. The sliders change the two terms separately, which is the quickest way to see what each one does: changing b slides the whole line up or down without tilting it, and changing m tilts it about the point where it crosses the vertical axis.