Find the decreasing interval, increasing intervals, and local maximum of the following function:

f(x) = 6x^3-18x^2-144x-7

The derivative is :

f'(x) = 18x^2-36x-144

The decreasing interval is (-2, 4)

The increasing intervals are (-inf, -2) and (4, inf)

I thought the local max was found by putting in the critical values ( in this case -2 and 4) into the original function. f(-2) = 161 and f(4)= -427.

So the local maximum = 161, right? When I submit this answer I'm told that it is incorrect. The rest of my answers are right, just not the local max.

Can anyone see where I am going wrong?

Thanks!