Question 678278
You take advantage of {{{(a+b)(a-b)=a^2-b^2}}}, by considering your denominator as either {{{(a+b)}}} or {{{(a-b)}}} and multiplying both numerator and denominator with the other paranthesis in the formula:

In your case the denominator is the {{{(a+b)}}}, thus you multiply with {{{(a-b)=(sqrt(3)-sqrt(6))}}}:

{{{((sqrt(3)-sqrt(6))/(sqrt(3)+sqrt(6)))*((sqrt(3)-sqrt(6))/(sqrt(3)-sqrt(6)))}}}

{{{(sqrt(3)-sqrt(6))^2/(3-6)}}}

{{{(3-2sqrt(18)+6)/(-3)}}}

{{{(9-6sqrt(2))/(-3)}}}

{{{-3+2sqrt(2)}}}