R.E. Lee entered into a 10-year lease in 2016 with Mr. Grant. The rent is $18,000 a year. Also, in April 2021, Mr. Lee repaired the kitchen at a cost of $4,500. Mr. Grant agreed to offer a rent credit for the kitchen repair, so Mr. Lee only paid a total rent of $13,500 to Mr. Grant for 2021 rent. In addition, in December 2021, he prepaid two months of 2022 rent totaling $3,000.How much rental income must Mr. Grant include in gross income in 2021?