2. The maximum of your model of tide height y=a cos b(x-c)+d is
a+d, and the mimium is -a+d as the maximum and minimum of cos
are +1 and -1. Set these equal to the average high and low tides
a+d = (3.52+3.96)/2 = 3.75
-a+d=(0.67+0.71)/2 = 0.69.
Use these to solve for a and d.
3 The interval t between the maximum and minimum of the model is:
t = 2 pi/b,
b = 2 pi / t,
so now compute the mean time between the high and low and low and
high tides and use that to solve for b.
4. The first maximum of the model occurs at 08:40, and this corresponds
to x-c=0, (where x is the time in decimal hours corresponding to 08:40),