To simplify the given expression we need to remove the radical from the denominator by rationalizing it.
To rationalize a radical expression first step is multiply both numerator and denominator of the expression by the conjugate of it's denominator.
So, conjugate of 6+√3 = 6-√3.
Hence,

=
=
Since (a+b)(a-b)= a^2-b^2
=

=

So, the answer is
