If the function y

If the function y = .021 t² + -1.3 t + 87 represents depth y vs. clock time t, then what is the average rate of depth change between clock time t = 7.5 and clock time t = 15? What is the rate of depth change at the clock time halfway between t = 7.5 and t = 15?

What function represents the rate r of depth change at clock time t? What is the clock time halfway between t = 7.5 and t = 15, and what is the rate of depth change at this instant?

If the function r(t) = .276 t + -1.2 represents the rate at which depth is changing at clock time t, then how much depth change will there be between clock times t = 7.5 and t = 15?

What function represents the depth?
What would this function be if it was known that at clock time t = 0 the depth is 130 ?