The tangent at $P$ to $y = x^2 - x^3$ meets the curve again at $Q$. Show that locus of midpoint of $PQ$ is $y=1-9x+28x^2-28x^3$ 
The tangent at a variable point $P$ of the curve $y = x^2 - x^3$ meets it again at $Q$. Show that the locus of the middle point of $PQ$ is $$y = 1 - 9x + 28x^2 - 28x^3$$

My approach 
$$y^\prime=2x-3x^2$$
Equation of tangent: $$2\ x_1 -3\ x_1^2=\frac{y-\ x_1^2+\ x_2^3}{x-\ x_1}$$
From the equation of tangent: 
$$(2\ x_1 -3\ x_1^2)(x-\ x_1)=y-\ x_1^2+\ x_2^3$$
Substituting the value of $y$ in the conic section we can find the value of $x$ but still it is getting complicated.

Given $f(x) = x^2-x^3$ the tangent line at $x_0$ is
$$
y = f(x_0) + f'(x_0)(x-x_0)\to y = \left(2 x_0-3 x_0^2\right) (x-x_0)-x_0^3+x_0^2
$$
Solving now for the intersection points we have
$$
y = x^2-x^3\\
y = \left(2 x_0-3 x_0^2\right) (x-x_0)-x_0^3+x_0^2
$$
with the feasible solution $x = 1-2x_0$ so the sought locus is given by
$$
(x, y) = \frac 12((x_0, f(x_0))+(1-2x_0,f(1-2x_0))) = \frac 12(1-x_0, 7(x_0^3-x_0^2)+2x_0)
$$
Attached a plot showing $f(x)$ in blue, $\frac 12(1-x_0, 7(x_0^3-x_0^2)+2x_0)$ in dashed red and the tangent line.

NOTE
Eliminating $x_0$ in the equations
$$
(x,y) =  \frac 12(1-x_0, 7(x_0^3-x_0^2)+2x_0)
$$
we obtain
$$
y = 1-9x+28x^2-28 x^3
$$
