I started here:
took the derivative to end up here:
and now I must solve for x. Or, find where the tangent line's slope is horizontal.
I'm stuck here, though I'm not sure if this was the right procedure.
$14x(x^2-3x+6)(\ln e) - 21(x^2-3x+6)(\ln e)$
The answer is $3/2$ I just don't know how to get there.