(a) Good.

(b) If the road must be closed for 3 hours, then the water creeps up on the road and rises for half of that time (1.5 hours) before high tide hits and the tide begins to recede...which it must do for the second half of the 3 hours. (We can make this assumption because the function used to represent the tide is vertically symmetric with respect to any maximum, or minimum for that matter). At any rate, what you have to do to find the height above sea level of the road is figure out the y-value of your function at the moment the water first creeps up onto the road. That should do it.