Find the critical points and determine if the function is increasing or decreasing on the given intervals.
$y = 3 x^4 + 6 x^3$

Left critical point: $c_1=$
Right critical point: $c_2=$
The function is:
on $(-\infty,c_1)$.
on $(c_1,c_2)$.
on $(c_2,\infty)$.