please explain

Assume there are two stocks, A and B, with BA 1.4 and BB = 0.8. Assume also that the CAPM model applies. (i) If the mean return on the market portfolio is 10% and the risk-free rate of return is 5%, calculate the mean return of the portfolios consisting of: a. 75% of stock A and 25% of stock B, b. 50% of stock A and 50% of stock B, c. 25% of stock A and 75% of stock B