Clicking on a particular solution step is the quickest way to review the logic which was employed.  It can also be helpful to begin a solution with 'a few steps' (use the 'Options' tab) and only increase the number of solution steps when the additional information is helpful (sometimes it can be confusing or overwhelming). 

The first step is to solve for a common denominator.  The two denominators are 3u^2 and 2u^3.  Since both '2' and '3' are primes they must be multiplied together for the common denominator, so '2/2' is multiplied by 3u^2 and '3/3' is multiplied by 2u^3.  Since 3u^2 "needs another factor of 'u'", then the factor (2/2) becomes (2u/2u) in order to complete the least common denominator.