Now all we have to do is to substitute for y = -3 m in the equation and solve for t, as simple as that.
t = T/2p cos-1(y/A)
Note that this equation will have multitude of answers, as there are many qs for
q = cos-1(y/A).
Also don’t forget that q should be in radian. But of course we can relate all the positive values of 't' to the real (physical) problem. The smallest positive value is when the sea level first reaches the y = -3 m, and the next smallest is when it reaches it for the second time etc.. If we designate those solutions as t1, t2, t3 and so on, it can be shown that t2 = T – t1 and t3 = T + t1, t4 = 2T – t1, t5 = 2T + t1 etc..