The depth vs

The depth vs. clock time function y = .019 t² + -1.4 t + 55 indicates the depth y of water in a certain uniform cylinder at clock time t.

At what average rate does depth changes between clock times t = 9.1 and t = 18.2?
What clock time lies midway between t = 9.1 and t = 18.2, at what rate is depth changing at this instant?

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

Evaluate this function at the clock time halfway between t = 9.1 and t = 18.2.

If the rate of depth change is given by dy/dt = .108 t + -1.8 represents the rate at which depth is changing at clock time t, then how much depth change will there be between clock times t = 9.1 and t = 18.2?

Give the function that represents the depth. What would this specific function be if at clock time t = 0 the depth is 100?