d$)$ Exxon has the following capital structure: the firm issued $6$ million shares of common stock with the stock price in c$),$ the firm also issued $1\mathrm{.}5$ million shares of preferred stock with $$4\mathrm{.}5$ preferred dividend per share, currently, Exxon has $$25$ millions in debts with interest rate as $6\mathrm{.}5\%.$ Suppose the current preferred stock price is $$6$ per share and the current common stock price is as in question $($c$),$ and the corporate tax rate is $25\%.$ What is the $($after$-$tax$)$ weighted average cost of capital for Exxon?