Answer
The work done by air resistance is $-3.79J$.
Work Step by Step
When there is air resistance, the energies are not conserved, so according to the work-energy theorem: $$W_{nc}=E_f-E_0$$ $$\Big(\frac{1}{2}mv_f^2+mgh_f\Big)-\Big(\frac{1}{2}mv_0^2+mgh_0\Big)=W_{air}$$ $$\frac{1}{2}m(v_f^2-v_0^2)+mg(h_f-h_0)=W_{air}$$
We take the starting point of the ball to be the moment when the ball is released and the finish point to be the moment it goes through the net.
We have $h_f=3.1m$, $h_0=2m$, $v_f=4.2m/s$, $v_0=7.2m/s$ and $m=0.6kg$, so $$W_{air}=-3.79J$$
