You invest $$2,000$ in a complete portfolio. The complete portfolio is composed of a risky asset with an expected rate of return of $16\%$ and a standard deviation of $30\%$ and a Treasury bill with a rate of return of $6\%$

$......$ $ of your complete portfolio should be invested in the risky portfolio if you want your complete portfolio to have a standard Deviation of $21\%$

Please i need the steps