=(4/(a+1)(5a-4) -(a+1)/(9(5a-4 )))*3(5a-4)/(a+7)=
(12/(a+1) -(a+1)/3) *1/(a+7) =((36 -(a+1)²)/(3a(a+1)) /(a+7)= (36 -a² -2a-1)(/3(a+1)(a+7)) = -(a² +2a -35)/(3(a+1)(a+7)) = -(a- 5)(a+7) /(3(a+1)(a+7) = (5-a)/(3(a+1)) .
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a² +2a-35 =a² - 5a+ 7a -35 =a(a -5 ) -7(a-5) =(a -5)(a+7).