A thermodynamic system is taken from an original state D to an...

Total work done by the gas from $D$ to $E$ to $F$=Area of $∆ DEF$

Work done during the process $DE$ is ${W}_{DE} = \dfrac{1}{2}\left(600 + 300\right)\left(5 - 2\right)J$

$= 1350J$

Work done during the process $EF$ is ${W}_{EF} = - 300 \times 3 = - 900J$

${W}_{DEF} = {W}_{DE} + {W}_{EF} = 450J$