You have no more money in your account, do you have insurance?
To get rid of the 3, you multiply the x/3 by 3 to get x by itself. But if you multiply one side you have to multiply the other. So x = 99
4/10 in simplest form is 2/5
6/20 in simplest form is 3/10
10/25 in simplest form is 2/5
The function for the height is

f(0) = 5 cos(0) + 7 = 5 + 7 = 12 (verified)
The period is T = 12 hours.
After 4 hours (t = 4), the height is

A graph of f(t) confirms the answer.
Answer: 4.5 ft