is $R_{s}=R_{1}+R_{2}=300 \mathrm{W}+500 \mathrm{W}=800 \mathrm{W}$

The error in equivalent resistance is given by

$\Delta \mathrm{R}=\left(\Delta \mathrm{R}_{1}+\Delta \mathrm{R}_{2}\right)=(3+4) \mathrm{W}=7 \mathrm{W}$

Hence, the equivalent resistance along with

error is $(800 \pm 7) \Omega$