Net work in a cyclic P-V process equals the signed area enclosed by the loop. The triangle has base $\Delta V=5-2=3$ m${}^3$ and height $\Delta P=300-100=200$ Pa.

Area $=\frac{1}{2}\times 3\times 200=300$ J. Cycle traversal A$\to$B$\to$C$\to$A is clockwise, so work done by the system is positive.

$W_{net}=+300$ J.