The depth of water at the entrance to a harbour t hours after low tide is D metres and can be modelled by the equation D= a+ b cos[c(t+d)] for the constants a,b,c and d

at t = 0, it is low tide and the depth is 2 metres.

At the next high tide, 6 hours later, the depth is 8 metres.

Find Exact values of a,b,c and given that all constants are positive..

any help with this i dont even have a start :S

thanks