1. The problem statement, all variables and given/known data The water depth in a harbour is 21m at high tide and 11m at low tide. One cycle is completed approximately every 12 hrs. Find an equation. 2. Relevant equations 3. The attempt at a solution The answer to this problem is y = 5sin 30 (t-3) + 16 A = (M - m) / 2 = (21-11) / 2 = 5 C = (M+m) / 2 = (21+11)/2 =16 Period is 12 hrs. K value would be 30 because 360 / 12. But I don't know why it is (t-3) I'm not sure where that came from.