Since E is a midpoint, we can set de and ef equal to each other to find the value of x.
6x = 3x + 6
3x = 6
x = 2
Then we can plug in x=2 and find that both de and ef are 12. Then since E is the midpoint of df, we can add de and ef to find the value of df.
12 + 12 = 24
