To rationalize the denominator, multiply by√5/√5 (which is 1).
15√6 (√5)
3√5 (√5)
This makes the √5 in the denominator 5.
15√6√5
3(5)
Multiply the √6 and √5 to get√30, which cannot be simplified, so you get
15√30
15
15/15 simplifies to 1, so the final answer is √30