You know the midpoint of AC is (4, 3). Let's call that point M.
You found the slope of the perpendicular which passes through
points B and D.
That would be 3.
Using the slope of 3 and the midpoint M(4, 3) and an endpoint B(x, y), we get:
Equating the numerators and denominators, we arrive at:
You can find the coordinates of D in a similar fashion.
Let coordinates of D be