A curve C has equation [math]y=x^3-2x^2-4x-1[/math] and cuts the y-axis at a point [math]P[/math]. The line [math]L[/math]is a tangent to the curve at [math]P[/math], and cuts the curve at the point [math]Q[/math]. Show that the distance PQ is [math]2\sqrt{17}[/math].