Can somebody me with my geometry?

The slope of AC is -0.4

Proof:
In triangles ABC and DBE,
∠DBE is common to both triangles.
AB = 2DB (D is the midpoint of the interval AB)
Also, BC = 2BE (E is the midpoint of the interval BC)

Thus triangles ABC and DBE are similar in the ratio 2:1

Since, they are similar, ∠BDE must equal ∠BAC (corresponding angles in similar triangles)

If ∠BDE = ∠BAC, DE must be parallel to AC (corresponding angles are equal along parallel lines)
Thus, the slope of AC = the slope of DE
Thus, the slope of AC is -0.4