If the function y = .025 t2 + -1.6 t + 60 represents depth y vs. clock time t, then what is the average rate of depth change between clock times t = 7.4 and t = 14.8?

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

If the rate of depth change is given by dy/dt = .057 t + -1.6 represents the rate at which depth is changing at clock time t, then how much depth change will there be between clock times t = 7.4 and t = 14.8?