Hello, Melaina,
See if this makes sense (ie., is correct).
Line D--E--F
E is the midpoint, so DE = EF
DE = 15-3x
EF = x+ 3
Since DE = EF,
15 - 3x = x + 3
-4x = -12
x = 3
If x = 3,
DE = 15 - 3*3, 15 - 9 = 6
EF = x+ 3, so EF = 3 + 3 = 6
That makes sense, DE = EF, since E is a midpoint.
EF = 6
I hope this helps,
Bob