Question

The water level at high tide was 10.3 feet; later, at low tide, it was 0.3 feet. Assuming the next high tide is exactly at 12 noon and that the height of the water is given by a sine or cosine curve, find a formula for the water level in Portland as a function of time, where y is the water level in feet, and t is the time measured in hours from midnight.

Answer 1

to be honest there isn't enough information here to answer the question.

info you do have is that the range is 10m
(note that a sin wave has range +/-1)

you know that the mean value is 5 (because the model is a sin wave)

you know that the peak of the tide is at 12 noon and you need to measure time from midnight

these give you the amplitude, phase and range of the function.

Unfortunately the question doesn't give you the period between tides.

As a rough approximation in fact there is about 12 hours between 2 high tides (and about 6 between high and low tide)

without making this assumption you cannot answer the question

remember the general formula is

H = a sin wt + phi