thank you for helping, however i have some doubts i want to clarify... may i ask why there are 3 values for (iii)? The method i used is find which quadrant sin positive lies in (ASTC method) and i got t=0.64905 and t=5.35095 (your first two values.) i just dont understand how you got the last one. Thank you:)

You're welcome.

There is nothing wrong with your method, just that if you sketch the graph (refer to my "bad sketching"), between the 2 values you found, the tide goes below 4 m where the ship cannot dock.

So we need to find the third value (which is second round first quadrant), and between 2nd and 3rd values, the tide is above 4 m, hence the ship could dock.

I hope the above explanation could clarify your doubts.

Alright I got it! thank u once again.