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(i)
$\frac{dx}{dt}=6t$

$\frac{dy}{dt}=6$

$\frac{dy}{dx}={\frac{dy}{dt}}\times {\frac{dt}{dx}}$

$\frac{dy}{dx}=\frac{1}{t}=0.4$

$t=\frac{5}{2}$

(ii)
At P, $t=p, \frac{dy}{dx}=\frac{1}{p}$
Equation of tangent: $y-6p=\frac{1}{p}(x-3p^{2})$
At D, $x=0, y=-3p+6p=3p$
Therefore, $D(0,3p)$.
Midpoint of $PD = (\frac{0+3p^{2}}{2}, \frac{6p+3p}{2})=(\frac{3p^{2}}{2}, \frac{9p}{2})$

As $p$ varies, $x=\frac{3}{2} p^{2}, y= \frac{9}{2}p$
We have that $p=\frac{2y}{9}$
Then, $x=\frac{3}{2}(\frac{2y}{9})^{2}$
Thus, $27x=2y^{2}$