I am studying for a test, and this is the question. Very Very Urgent.

At an ocean port, the water has a max depth of 4m above the mean level at 8 am, and the period is 12.4 h. Assuming that the relation between the depth of the water and time is a sinusoidal function, write an equation for the depth of the water at any time t.

I am terrible at modeling these equations. Any tips ?