Answer:

Explanation:
As it is a fluctuating scenario, where the value repeats itself after some time, it must be represented by periodic function.
As the value at the beginning is a non zero value, we should use a Cosine function.
The general cosine function is,

where,
a = amplitude of motion,
p = period ,
b = vertical displacement.
The water level at high tide was 9 feet and 1 foot at low tide. So,


The next high tide is exactly 12 hours later. So period or p = 12
Putting all the values,
