If the risk-free rte is 7%, the expected return on the market is 10%
and the expected return on Security J is 13%, what is the beta of
security J

Hi!!

To solve this use the CAPM formula:
r = Rf + Beta x (Rm - Rf) 
where 
r is the expected return rate on a security; 
Rf is the "risk-free rate"; 
Rm is the return rate of a market benchmark, like the S&P 500. 

According to the problem statement you know r, Rf, and Rm; so you only
need to isolate Beta; after doing the calculations you will find that
Beta = 2.

For references see:
"Capital Asset Pricing Model (CAPM)":
http://www.moneychimp.com/articles/risk/classes.htm

"CAPM - Capital Asset Pricing Model":
http://www.valuebasedmanagement.net/methods_capm.html


Search strategy to find references:
I used the following keyword at Google.com:
CAPM formula


I hope this helps you. Feel free to request for a clarification if you need it.


Regards,
livioflores-ga