Mary Ellen sold property for $30,000 cash and paid a sales commission of $2,100. The buyer assumed the existing mortgage of $13,000 owed by Mary Ellen on the property. Mary Ellen purchased the property for $18,000, and she has invested an additional $3,000 in it during the time she held it. What is Mary Ellen's total realized gain?

a) $ 6,900 b) $ 9,000 c) $19,900 d) $22,000