Surface area $S= 4\pi r^2 = 4 \pi (2.1)^2 = 55.4\;cm^2$

$\Rightarrow \large\frac{\Delta S}{S}$$ = 2 \large\frac{\Delta r}{r}$$ \rightarrow \Delta S = 2 \large\frac{\Delta r}{r}$$S$

$\Rightarrow \Delta S = \large\frac{2 \times 0.5 \times 55.4}{2.1}$$ = 26.4 \;cm^2$

$\Rightarrow S = 55.4 \pm 26.4 cm^2$