Решение и ответ:

[tex]\displaystyle \frac{{3x+xa-3y-ay}}{{9x-9y}}=\frac{{3\left( {x-y} \right)+a\left( {x-y} \right)}}{{9\left( {x-y} \right)}}=\frac{{\left( {3+a} \right)\left( {x-y} \right)}}{{9\left( {x-y} \right)}} =\frac{{3+a}}{9}[/tex]