Hello, rom!
I will assume that a $\displaystyle t$ was omitted from the function.
1. Entering the harbour
The depth $\displaystyle y$ in metres above a horizontal mark on a harbour wall is given by:
. . $\displaystyle y \:=\:3\sin\left(\frac{\pi}{12}t\right)$ .where $\displaystyle t$ is the time in hours after midday on Monday.
(a) Plot the graph of $\displaystyle y \:=\:3\sin\left(\frac{\pi}{12}t\right)$ for $\displaystyle 0 \leq t \leq 36$
(b) The rate of change of depth of water in the harbour is given by $\displaystyle \frac{dy}{dt}$
Find $\displaystyle \frac{dy}{dt}$and calculate the rate of change of depth of water when $\displaystyle t = 19$
(c) List the coordinates of all the stationary points on the graph.
(d) A luxury liner can only enter the harbour when the depth of water
is at least 1.5 metres above the horizontal mark.
At what times during the week can the liner enter the harbour?
(a) The graph looks like this: Code:

3 + * *
 * : * * : *
 * : * * : *
* : * * : *
 : 18 :
*+*+*+*
 6 12 : 24 30 36
 * : *
 * : *
 * : *
3+ *

(b) The derivative is: .$\displaystyle \frac{dy}{dt}\:=\:3\cos\left(\frac{\pi}{12}t\right )\cdot\frac{\pi}{12} \:=\:\frac{\pi}{4}\cos\left(\frac{\pi}{12}t\right) $
When $\displaystyle t = 19,\;\frac{dy}{dt} \:=\:\frac{\pi}{4}\cos\left(\frac{19\pi}{12}\right ) \:\approx\: 0.2$ metres per hour (rising).
(c) On the graph, we see that the startionary points (horizontal tangents):
. . are at: .$\displaystyle (6,\,3),\:(18,\,3),\:(30,\,3)$
(d) When is $\displaystyle y \geq 1.5\,?$
The first time $\displaystyle y = 1.5$, we have: .$\displaystyle 3\sin\left(\frac{\pi}{12}t\right) \:=\:1.5\quad\Rightarrow\quad \sin\left(\frac{\pi}{12}t\right) \:=\:0.5$
. . Then: .$\displaystyle \frac{\pi}{12}t\:=\:\frac{\pi}{6}\quad\Rightarrow\ quad t = 2$
The next time $\displaystyle y = 1.5$ is $\displaystyle t = 10$
On the graph, we see that $\displaystyle y \geq 1.5$ for: $\displaystyle 2 \leq t \leq 10$
. . and it happens again for: $\displaystyle 26 \leq t \leq 34$
Therefore, the liner can enter the harbour on Monday from 2 pm to 10 pm
. . and on Tuesday from 2 pm to 10 pm.