On a particular day, the power used in a particular state (in thousands of megawatts) could be approximated by the function P(t) = -0.00688813 +0.191812 -0.7085t + 20.74, where t is the number of hours since midnight, for 0 st 24. Find any relative extrema for power usage, as well as when they occurred. Find the derivative of P(t). P'(t)=