Question 62531
  

   {{{(x^2-9)/(x^2+7x+12)}}}  =  {{{(x+3)*(x-3)/((x+3)*(x+4))}}}  =  {{{cross((x+3))*(x-3)/(cross((x+3))*(x+4))}}} = {{{(x-3)/(x+4)}}}  Answer.  
  
In such problems, first factorise the numerator and denominator.  Then cancel the common factors in the numerator and the denominator.  You get the answer.  Try some for yourself and see how easy it is. All the best.
  
gsm