Shawn Bixby borrowed $28,000 on a 210-day, 12% note. After 90 days, Shawn paid $3,100 on the note. On day 129, Shawn paid an additional $5,100. Use ordinary interest. a. Determine the total interest use the U.S. Rule. (Do not round intermediate calculations. Round your answer to the nearest cent.) Total interest $ b. Determine the ending balance due use the U.S. Rule. (Do not round intermediate calculations. Round your answer to the nearest cent.) Ending balance due $