The answer is 5 hours and I had to graph 2 cycles of the function to find that out. Are there any other approaches or methods to solving this other than drawing?Because of the tide, the depth of the water in a harbor is modeled by the equation d=-3cos(pi/6)t + 6, where d represents the depth of the water in meters and t represents the number of hours after midnight/ (i.e. t=0 means midnight, t=1 means 1 A.M., and so on.)

b) Surfing is allowed between 8 A.M. (08:00 hrs) and 7 P.M. (19:00 hrs), but only when the depth of the water is 6m or more. For how many hours is surfing allowed in one day?

Thank you in advance.