(a)
For $x = p,\ y = ap^{2}bp + c$
And for $x = q,\ \ \ y = aq^{2} + bq + c$
Slope $= \frac{aq^{2} + bq + c - ap^{2} - bp - c}{q - p}$
$= a(q + p) + b$
$\frac{dy}{dx} = 2ax + b = a(q + p) + b$
(according to the equation)
$\therefore\ \ \ x = \frac{q + p}{2}$