-6√63 - 9√28 + 2√14

Factor 63 as 9*7 and then as 3²·7, so 63 contains a perfect square factor

Factor 28 as 4*7 and then as 2²·7, so 28 contains a perfect square factor

Factor 14 as 2*7 but 14 does not contains a perfect square factor,
so we just leave it as 14

-6√3²·7 - 9√2²·7 + 2√14

We can take 3² out of the first square root as 3 
in front and take 2² out of the second square root 
as 2 in front, like this:

-6·3√7 - 9·2√7 + 2√14

-18√7 - 18√7 + 2√14

The first two terms combine as -36√7

-36√7 + 2√14

That's the simplest radical form.

Edwin