How do i calculate betas?

#1
I have the rate of returns and standard deviations for 2 companies and I have the value of ASX200 on tat particular date. I need to find the betas for both the companies.

The fomula for Beta is Cov(ra,rp)/Var(rp).
where ra measures the rate of return of the asset and rp measures the rate of return of the portfolio of which the asset is a part.

But I dont have the return for the portfolio.. wat should i do? Should i assume tat my portfolio consists of both the stocks in equal proportion. Also I have the risk free return.. any idea where that fits in...thanx

Also if u have the excel sheet formula to find the Beta, it would be really appreciated.