how do you find rate of change?

In graphing the rate of change is measured by the gradient.

In calculus the instantaneous rate of change at any specific point on the curve is given by the gradient of the tangent at that point which IS the first derivative.  :)

The rate of change is the amount of change in one measurement divided by the amount of change in another measurement.

For example, speed is a rate of change:  the change of distance divided by the change of time; or, how far you went divided by how long it took you.

For example, if you are on an interstate and you pass mile marker 100 at 2:00 and you pass mile marker 240 at 4:00, you traveled 140 miles (240 - 100) in 2 hours (4:00 - 2:00), so your speed (rate of change) is 140 miles / 2 hours or 70 miles per hour.

Another example is slope: slope is the change in y-values divided by the change in x-values. It's the rate of change of y (rise) as x changes (run).