Value of x of which a slope is undefined for a parametric graph.

Question

For what values of $x$ is the slope undefined for the graph

$$x=8-t^3$$

$$y=t^2-6t$$

The slope should be undefined when $\frac {dx}{dt}=0$.

$$\frac {dx}{dt}=-3 t^2$$

$$-3t^2=0$$

$$t=0$$

When $t=0$, $x=8$ so the $x$ value should be $8$, but the question was marked incorrect. What is incorrect in this?

Answer 1

Answer was x=8, question incorrectly marked.