Answer
See the graph below.
Work Step by Step
We know that the sound wave moves from the sonar until it hits the bottom of the ocean and then bounces up again to the sonar.
So the distance traveled by the wave during the time $\Delta t$ is twice the depth of the ocean.
Hence the depth is given by
$$v=\dfrac{d}{\frac{1}{2}\Delta t}$$
$$d=\frac{1}{2}v\Delta t$$
where $v$ is the sound speed in the water which is about 1.5 km/s.
$$d=\frac{1}{2}(1.500)\Delta t=\boxed{0.750\Delta t}$$
Using the given data to find the depth at each given point;
\begin{array}{|c|c|c|c|}
\hline
x\;{\rm (km)}& d\;{\rm(km)}&\Delta t\;{\rm (s)}\\
\hline
0 & \color{red}{\bf 4.5} &6\\
\hline
20& \color{red}{\bf 3} &4\\
\hline
40 & \color{red}{\bf 3} &4 \\
\hline
45 & \color{red}{\bf 6} &8 \\
\hline
50& \color{red}{\bf 3} &4\\
\hline
60& \color{red}{\bf 1.5} &2 \\
\hline
\end{array}
Now we can easily draw the graph but remember to put the data of the depth in negative since the author needs it under the surface of the ocean which is considered as $y=0$
