Question 1116830
Maybe you mean those rational expressions are  2x/(x^2-81) and (x-1)/(9x-81).


{{{2x/(x^2-81)}}}
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Denominator is  {{{(x-9)(x+9)}}}.


{{{(x-1)/(9x-81)}}}
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Denominator is  {{{9(x-9)}}}.



Simplest common denominator would be {{{9(x-9)(x+9)}}}.



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{{{(2x/(x^2-81))(9/9)=highlight(18x/(9x^2-729))}}}



and


{{{((x-1)/(9x-81))((x+9)/(x+9))}}}--------you can finish this one.