The outside is △ABC, and the inside is △DEF.

According to the meaning of the question:

A, d, E*** line, AD=DE, d is the midpoint of AE.

Lines b, e, F***, BE=EF, e is the midpoint of BF.

C, f, D*** lines, BF=FD, and f is the midpoint of CD.

So:

CF=FD, we can get AD-AF=AF-AC, and simplified AF =1/2 * AD+1/2 * AC ①.

BE=EF, AE-AB=AF-AE, and simplified AF=2AE-AB=4AD-AB②.

By who? ①*8-②? Get 7AF=AB+4AC.

Simplified AF= 1/7*AB+4/7*AC.

Get the answer b