Answer
$D(t) = 7.0+5.0~cos[\frac{2\pi}{12}~(t-6.75)]$
Work Step by Step
The average depth is 7.0.
The amplitude of the graph is 5.0.
The period is 12 hours.
A high point occurs at 6:45 am, which is 6.75 hours after midnight.
We can use these values to write a function that models the depth:
$D(t) = 7.0+5.0~cos[\frac{2\pi}{12}~(t-6.75)]$
