A company's stock is currently selling for $$15\mathrm{.}25$ per share, and its noncallable $$1,000\mathrm{.}00$ par value, $20-$year, $9\mathrm{.}00\%$ bonds with semiannual payments are selling for $$930\mathrm{.}41.$ The firm's tax rate is $21\%.$

What is the best estimate of the after$-$tax cost of debt?