Assume a firm has $20-$year, $8\%$ coupon bonds with a current market yield of $8\mathrm{.}06\%.$ The firm's combined federal and state corporate tax rate is $28\%.$ What is the firm's after$-$tax cost of debt? Enter your answer in percent, leaving off the $"\%"$ and rounding your answer to the nearest $0\mathrm{.}01\%.$