The formula has 4 coefficients you need to find, a, b, c, d. To do that you need 4 "conditions" or 4 equations. You are given:

1. When t= 0, the depth is 2: 2= a+ bcos(d)

2. 6 hours later, when t= 6, the depth is 8: 8= a+ bcos(6c+ d).

3. When t= 0, the depth is minimum. The minimum of cos(x) is -1 and occurs when : 2= a- b and .

4. When5 t= 6, the depth is maximum. The maximum of cos(x) is 1 and occurs when : 8= a+ b and .