E is the midpoint

Question

E is the midpoint of DF,DE=15-3x, and EF=x+3,then EF= What the answers are 1, 3, 6, 12

Answer 1

15-3x = x + 3 = EF

4x=12

x = 12/4 = 3

EF = x+3 = 3+3 = 6

Answer 2

If E is the midpoint then DE = EF, so you can equate the two expressions for those lengths and solve for x:

15 - 3x = x + 3

12 = 4x

3 = x

substitute 3 for x in the expression for EF: x + 3 = 3 + 3 = 6