First find the secant line. The slope of the secant line through 
(when 
) and 
(when 
) is the average rate of change of 
over the interval ![[-1,2]](https://tex.z-dn.net/?f=%5B-1%2C2%5D)
:
The tangent line to will have a slope determined by the derivative:
Both the secant and tangent will have the same slope when
, or when 
.
The tangent line to
Both the secant and tangent will have the same slope when