If the shark in the net is not moving in the up/down direction, the magnitude of the tension in the cable is the magnitude of the weight of the shark. That is $\displaystyle mg$. But as there is an acceleration (the shark is getting pulled), the tension in the cable is $\displaystyle m(g+a)$, where a is the acceleration of the net.
So work out
The net is steadily accelerated from rest to 1.0 m/s in a time interval of 2.55 s.

in order to get the acceleration of the shark. Once you get it, plug it into the formula $\displaystyle T_{cable}=m(g+a)$ and you're done.