Hello, i need help with this question. A guaranteed thumps up for the correct answer.

A portfolio consists of two securities: a 90-day T-bill and the S\&P/TSXComposite. The expected return on the T-bill is 4.5%. The expected return on the S\&P/TSX Composite is 12% with a standard deviation of 20%. What is the portfolio standard deviation if the expected return for this portfolio is 15% ? 12.00% 8.13% 28.00% 16.80%