Please explain/show why the standard deviation of the portfolio is 3.9

Table 11.6 Rate of return assumptions for two stocks Table 11.8 Rates of return for two stocks and a portfolio with 75% invested in the auto stock and 25% in the gold stock * Portfolio return = (-75 x auto stock return) + (.25x gold stock return). Rate of Return (%) Rate of Return (%) Scenario Auto Stock Probability Auto Stock Gold Stock Scenario Portfolio Return (%) Probability Recession 1/3 -8 +20 Recession 1/3 -8 - 1.0 Normal 1/3 +5 +3 Normal 1/3 +5 +4.5 Boom 1/3 +18 -20 Boom 1/3 +18 +8.5 Expected return 5 I 4 Variance 112.7 268.7 15.2 Standard deviation 10.6 16.4 3.9 . Show why standard deviation of portfolio is 3.9. Hint: After calculate Covariance, use SD= (W SD +W SD + 2 *W *W Cov, b) b Gold Stock +20 +3 -20