If the function y

If the function y = .019 t² + -2.1 t + 80 represents depth y vs. clock time t, then what is the average rate of depth change between clock time t = 15.1 and clock time t = 30.2? What is the rate of depth change at the clock time halfway between t = 15.1 and t = 30.2?

What function represents the rate r of depth change at clock time t? What is the clock time halfway between t = 15.1 and t = 30.2, and what is the rate of depth change at this instant?

If the function r(t) = .017 t + -1.5 represents the rate at which depth is changing at clock time t, then how much depth change will there be between clock times t = 15.1 and t = 30.2?

What function represents the depth?
What would this function be if it was known that at clock time t = 0 the depth is 90 ?