If the function y = .023 t2 + -1.1 t + 50 represents depth y vs. clock time t, then what is the average rate of depth change between clock times t = 6.8 and t = 13.6?

What function represents the rate r of depth change at clock time t?

If the rate of depth change is given by dy/dt = .168 t + -1.7 represents the rate at which depth is changing at clock time t, then how much depth change will there be between clock times t = 6.8 and t = 13.6?