Answer-

Solution-
Triangle Midpoint Theorem-
The line segment connecting the midpoints of two sides of a triangle is parallel to the third side and is congruent to one half of the third side.
As given in the question,
AD = DB, so D is the mid point of AB,
FA = FC, so F is the mid point of AC.
Hence, DF is the line joining the midpoints of AB and AC.
Applying Triangle Midpoint Theorem,




Therefore, the side length of BC is 8 units.