The dot between the Na²Co³ and NH²O is plus sign

✅ Correct.

Your understanding that the dot in $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$ signifies that the molar mass of the water molecules is added to the molar mass of $\text{Na}_2\text{CO}_3$ to get the total molar mass of the hydrate is correct. The previous calculation for part c) already followed this principle.