Rumolt Motors has $33$ million shares outstanding with a share price of $ $15$ per share. In$$ addition, Rumolt has issued bonds with a total current market value of $ $138$ million. Suppose$$ Rumolt's equity cost of capital is $12\%,$ and its debt cost of capital is $7\%.$

a$.$ What is$$ Rumolt's pre$-$tax$$ WACC?

b$.$ If$$ Rumolt's corporate tax rate is $21\%,$ what is its$$ after$-$tax WACC?