For $f(x)=4x^3-6x^2-72x+30$, the function is decreasing on the interval

1. $f'(x)=12x^2-12x-72=12(x-3)(x+2)$. 2. $f'(x)=0$ at $x=-2,3$. 3. On $(-2,3)$: $(x-3)<0$, $(x+2)>0$, so $f'(x)<0$ (decreasing). 4. On the outer intervals $f'(x)>0$ (increasing), ruling out A and B. _Source: syllabus Mathematics Ch 6 "Application of Derivatives", p.9_