Amphoteric means it behaves as both acid and base. ${\mathrm{Na}}_2\mathrm{O}$ is basic oxide and $\mathrm{ZnO}$ will act as acidic oxide in reacting with this oxide. ${\mathrm{CO}}_2$ is acidic oxide and $\mathrm{ZnO}$ will act as basic oxide in reacting with this oxide. So, with ${\mathrm{Na}}_2\mathrm{O}$, it acts as an acid and with ${\mathrm{CO}}_2$, it acts as a base.