Explanation:
since it is a mid-segment, DGH and DEF are similar triangles with the constant scaling factor of 2 for the lengths of all sides and other lines in the triangle.
e.g.
DE = 2 × DG
DF = 2 × DH
and therefore,
28 = 2 × GH = 2× (x - 3)
14 = x - 3
x = 17
HF = x - 7 = 17 - 7 = 10
DH = HF = 10
so,
y + 8 = 10
y = 2
and DF = 10 + 10 = 20