In late 2014, Randy and Rachel Erwin paid $7,000 in legal fees, adoption fees, and other expenses directly related to the adoption of an infant son, Jameson. The adoption becomes final in 2015, and the Erwins they pay an additional $8,000 in adoption fees. Their 2015 AGI amounts to $135,000.

a. Determine the amount of the couple's 2015 adoption tax credit.

b. How does your answer change if the Erwins' 2015 AGI is $210,000?