The water at a local beach has an average depth of one meter at low

The water at a local beach has an average depth of one meter at low tide and an average depth at high tide of 8m. One cycle of the tides takes approximately 12 hours. This periodic motion can be modelled by the function d(t)=3.5cos [6 t]+4.5where d(t) represents the depth of the water, in metres, at a time t, in hours. This equation assumes that the water level is at high tide at time zero (midnight). Many people dive into the water from a nearby dock. If the water must be at least 3m deep to dive safely, when during the first 24 hour period (in hours) would it be safe to dive off the dock? Please give your answer in all ranges of hours possible.