On July 17, 2019, the power used in New York state (in thousands of megawatts) could be approximated by the function P(t) = -0.006785t³ + 0.1858t² - 0.6549t + 20.62, where t is the number of hours since midnight, for 0 ≤ t ≤ 24. Find any relative extrema for power usage, as well as when they occurred.