When quantities are added or subtracted, their absolute errors add up. For resistors in series, the equivalent resistance is $R_{eq} = R_1 + R_2$. Mean value of $R_{eq} = 100 \Omega + 200 \Omega = 300 \Omega$. The maximum possible absolute error in the sum is the sum of the individual absolute errors: $\Delta R_{eq} = \Delta R_1 + \Delta R_2 = 3 \Omega + 4 \Omega = 7 \Omega$. Therefore, the equivalent resistance is $(300 \pm 7) \Omega$.