can I please have an answer to this question, with the formula please?!

20. Go to www.mhhe.com/blnn and link to Chapter 7 materials, Where you will find a spreadsheet with monthly returns for GM, Ford, and Toyota, the SSCP 500, and Treasury bills. (LO 7- 1) 4. Estimate the index model for each rm over the full ve-year period. Compare the betas of each rm. 5. Now estimate the betas for each rm using only the rst two years of the sample and then using only the last two years. How stable are the beta estimates obtained from these shorter subperiods