If the water must be at least 3m deep to dive safely, when during the daylight hours would it be safe to dive off the dock

Question

The water at a local beach has 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[𝛑/6 t] where 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 a high tide at time zero. 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 daylight hours would it be safe to dive off the dock?

Answer 1

Easiest way to answer this is to plot it.

The Cosine wave intersects the minimum depth 3 at (11,3) and (13,3)

which equates to 1 to 3 pm or 1 to 3 am time frames.