Let $a$ and $b$ respectively be the points of local maximum and local minimum of the function $f(x)=2 x^3-3 x^2- 12 x$. If $A$ is the total area of the region bounded by $y=f(x)$, the $x$-axis and the lines $x=a$ and $x=b$, then 4A is equal to $\_\_\_\_$ .
