Use the difference quotient to find the slope of the secant line. The slope of the secant line is your average rate of change of your function.

You find the slope of the tangent line by taking the derivative of your function. You can also find it by using the difference quotient from the secant line by taking it at the limit of the point of tangency. The slope of the tangent line is the instantaneous rate of change at that exact point on your curve.