Average rate of change = (y2 - y1) / (x2 - x1)

This is the slope of the line joining the two points. If x1 = x2, the value is undefined because the denominator is zero.

The sign tells you whether y is increasing or decreasing as x increases.

A positive value means the points rise from left to right. A negative value means they fall from left to right.

Using the example values, the points (0, 2) and (5, 17) give Delta y = 15 and Delta x = 5, so the average rate of change is 3.