A thermodynamic process is carried out from an original state $D$to an intermediate state $E$ by the linear process shown in figure. The total work is done by the gas from$D$ to$E$ to $F$ is

Total work done by the gas from $D$ to $E$ to $F$is equal to the area of $∆DEF.$

The area of $∆DEF =\frac{1}{2}DF \times EF$

Here, $\hspace{0.17em}DF$ change in the pressure $=600-200=400 N-m^{-2}$

Also, $EF$$=$change in the pressure $=7m^3-3m^3=4m^3$

Area of $△DEF = \frac{1}{2}\times 400\times 4 = 800 J$

Thus the total work done by the gas from $D$ to $E$ to $F$ is $800 J$