Question 879800
Multiply top and bottom by {{{2+sqrt(7)}}} (this is the conjugate of the denominator)



{{{9/(2-sqrt(7))}}}



{{{(9(2+sqrt(7)))/((2-sqrt(7))(2+sqrt(7)))}}}



{{{(9(2+sqrt(7)))/((2)^2-(sqrt(7))^2)}}}



{{{(9(2+sqrt(7)))/(4-7)}}}



{{{(9(2+sqrt(7)))/(-3)}}}



{{{-3(2+sqrt(7))}}}



{{{-6-3sqrt(7)}}}



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So {{{9/(2-sqrt(7)) = -6-3sqrt(7)}}}