Rumolt Motors has $36$ million shares outstanding with a price of $$21$ per share. In addition, Rumolt has issued bonds with a total current market value of $$667$ million. Suppose Rumolt's equity cost of capital is $9\%,$ and its debt cost of capital is $5\%.$

a$.$ What is Rumolt's pretax weighted average cost of capital?

b$.$ If Rumolt's corporate tax rate is $20\%,$ what is its after$-$tax weighted average cost of capital?