You cannot have radicals in the denominator. So your first step would be to multiply both the top and bottom by √6.
12(√6) / 5(√6)(√6)
The (√6)(√6) is like doing (√6)². Squaring something that is square-rooted just cancels each other out. This leaves you with just 6. So, simplify and get this:
12√6 / 5(6) = 12√6 / 30
12 and 30 have a greatest common factor of 6 so you divide both of these numbers by 6. Your answer becomes:
(2√6) / 5