1. The height is given by . Normally there would be a phase variable, but we are given that there is a maximum at t=0, so the phase must be zero. The difference between high and low tide is , so and since the max is 8, (I could have also used the min). Since one cycle is 12 hours, . So the equation is:
The other two problems just work with this equation. For (2), you set t=2 and find h(t). For (3), you need to know what range of t makes h(t) is greater than or equal to 3.
Post again in this thread if you're still having problems.