For an isobaric reversible expansion of an ideal gas,

The work done, $\mathrm{W}=-{\mathrm{P}}_{\mathrm{ext}}({\mathrm{V}}_2 -{\mathrm{V}}_1)$

But, ${\mathrm{P}}_{\mathrm{ext}}={\mathrm{P}}_1={\mathrm{P}}_2$ for an isobaric process.

From the ideal gas equation,

$\mathrm{V} =\frac{\mathrm{nRT}}{\mathrm{P}}$

So, the work done $\mathrm{W}=-\mathrm{P}\left(\frac{{\mathrm{nRT}}_2}{\mathrm{P}} -\frac{{\mathrm{nRT}}_1}{\mathrm{P}}\right)=-\mathrm{nR}({\mathrm{T}}_2 -{\mathrm{T}}_1)$

Given: $\mathrm{n}=3; {\mathrm{T}}_1=298 \mathrm{K} \text{and} {\mathrm{T}}_2=400 \mathrm{K}; \mathrm{R}=8.314 \mathrm{J} {\mathrm{K}}^{-1} {\mathrm{mol}}^{-1}$

$\mathrm{W}=-3\times 8.314\times \left(400-298\right)=-2544 \mathrm{J}$

$\mathrm{W}=-2.54 \mathrm{kJ}$