Multiply
(-4x-3y) (5y-12z) =
(x2 + 8x + 7) ÷ (x + 6)=
Use the "distributive law", twice;
-4x(5y- 12z)- 3y(5y- 12z) is one use of it
-4x(5y)- 4x(12z)- 3y(5y)- 3y(-12z) is the second. Multiply those.
(x2 + 8x + 7) ÷ (x + 6)=[/QUOTE]
So this is $\displaystyle \frac{x^2+ 8x+ 7}{x+ 6}$. It should be obvious that x+ 6 is NOT a factor of the numerator (6 is not a factor of 7) so you cannot reduce it any more.