Don't start out multiplying by the entire denominator over itself.
Instead, first get the prime factorization of 18 as 2*3²



To get a perfect square under the radical,
we need to multiply what's under there by another 2 and also
by another x, so we multiply by  which is just 1 in value.



Multiplying under the radicals in the denominator:



Now we will no longer have a radical in the
denominator because we can take the square roots
of all those squares





Then the 6 divides into the 12 and we have:



Edwin