What formula is used? Can you give me the details?

+ 42.+ + An analyst estimates the index model for a stock using regression analysis involving total returns. The estimated the intercept in the regression equation is 6% and the B is 0.5. The risk-free rate of return is 12%. The true of the stock is + + t A +0%. B. +3%. C. 6%. D. +9%. 6% = a + 12% (1 -0.5); a = 0%.+