Clicking on a particular solution step is the quickest way to review the logic which was applied.  It is also good to change the number of solution steps (click on the 'Options' tab) in order to look at the simplification through different perspectives.  Typically it is best to start with 'a few steps' and only increase the number of solution steps when that information is "enlightening". 

The addition of a fraction to a another term begins with finding a common least denominator.  In that equation the left side has a denominator of '3', so multiplying the second term by '3/3' allows all the terms to become a numerator with a denominator of '3'.