Find the average rate of change of y with respect to x from P to Q$.$ Then compare this with the instantaneous rate of change of y with respect to x at P by finding m Subscript tan at P$.$

y equals $8$ minus x cubed$$; Upper P left parenthesis $3$ comma negative $19$ right parenthesis$,$ Upper Q left parenthesis $3\mathrm{.}1$ comma negative $21\mathrm{.}791$ right parenthesis