x^2+4x/x+3 * x^2-9/x^2+x-12  

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wow  I really wishd i could help you here but i think  u missed out tons of brackets , which makes things amazingly hard for me.But im assuming the question is as follows:(x^2+4x)/(x+3) * (x^2-9)/(x^2+x-12)=(x^2+4x)/(x+3) * (x+3)(x-3)/(x^2+4x-3x-12)=(x^2+4x) * (x-3)/(x+4)(x-3)=x(x+4) * 1/(x+4)=x