Find all values of $c$ such that there exists a line that intersects the graph of $f(x) =x^4+9x^3+cx^2+9x+4$ in four distinct points.
I considered the derivative $f'(x)=4x^3+27x^2+2cx+9$. If it could be written in the form $4(x^2+r^2)(x-s)$ or $4(x-t)^3$, then there's only one local minimum. The first yields $r^2=1/3,s=-27/4,c=2/3$ and the second yields no solution. If $c=2/3$, then $f$ is strictly increasing for $x>-27/4$ and strictly decreasing for $x<-27/4$. Therefore if $c=2/3$ then there wouldn't exist such line.
However, the correct answer to this problem is $c<243/8.$ What's wrong with the claim $c\neq2/3$?