Marginal Incorporated $($MI$)$ has determined that its before$-$tax cost of debt is $6\mathrm{.}0\%$ for the first $$134$ million in bonds it issues, and $10\mathrm{.}0\%$ for any bonds issued above $$134$ million. Its cost of preferred stock is $15\mathrm{.}0\%.$ Its cost of internal equity is $18\mathrm{.}0\%,$ and its cost of external equity is $21\mathrm{.}0\%.$ Currently, the firm's capital structure has $$276$ million of debt, $$30$ million of preferred stock, and $$294$ million of common equity. The firm's marginal tax rate is $35\%.$ The firm's managers have determined that the firm should have $$89$ million available from retained earnings for investment purposes next period. What is the firm's marginal cost of capital at a total investment level of $$236$ million?