Question

here is my question i want to know what they mean by show please :) The depth of water D at the end of Brighton pier t hours after low tide is given by the rule D = p − 2 cos (rt), where p and r are suitable constants.
At low tide (t = 0) the depth is 2 metres; at high tide, which occurs 8 hours later, the depth is 6 metres. ’
Show that r=π\8 and p=4