The middle line is equal to 1/2(top + bottom)
3x + 6 = 1/2(5 + 7x + 5)
3x + 6 = 1/2(7x + 10)
Expand the right side:
3x + 6 = 7x/2 + 5
Subtract 6 from both sides:
3x = 7x/2 -1
Multiply both sides by 2
6x = 7x -2
Subtract 7x from both sides
-x = -2
Multiply both sides by -1
X = 2
Now you have a value for x solve for EF:
EF = 3(2) + 6 = 6 + 6 = 12
Answer: EF = 12