%%\dfrac{c^2+c-20}{-c^2+c+30}%%

c2+c20c2+c+30=(c4)(c+5)(c+5)(c+6)=c4c+6=c4c6 \dfrac{c^2+c-20}{-c^2+c+30}= \dfrac{(c-4)\cdot(c+5)}{(c+5)\cdot(-c+6)}= \dfrac{c-4}{-c+6}=- \dfrac{c-4}{c-6}