please explain the calculations!!! note it says NO EXCESS capacity

Darrin's Auto Northern Division is currently purchasing a part from an outside supplier. The company's Southern Division, which has no excess capacity, makes and sells this part for external customers at a variable cost of $9 and a selling price of $21. If Southern begins sales to Northern, it (1) will use the general transfer-pricing rule and (2) will be able to reduce variable cost on internal transfers by $3. On the basis of this information, Southern would establish a transfer price of: Multiple Choice None of the answers is correct. $21. $6. $18. $9