Let the normal at the point \( P \) on the parabola \( y^2 = 6x \) pass through the point \( (5, -8) \). If the tangent at \( P \) to the parabola intersects its directrix at the point \( Q \), then the ordinate of the point \( Q \) is:
\( -3 \)
\( -\dfrac{9}{4} \)
\( -\dfrac{5}{2} \)
\( -2 \)
