It's 7.4 because the 3 doesn't round the 4 up.
M<ABC = m<EBD = 36 (vertical angles)
78 - x + 36 + 3x - 10 = 180 (straight angle)
Now solve for x
78 - x + 36 + 3x - 10 = 180
2x + 68 = 180
- 68 -68 (subtract 68 on both sides)
2x = 58
x = 29
5a+6b would be the simplified answer
There has to be something in between the two numbers, for example 7 x 7 or 7+7 not just 7 7. Without the equation it would be highly impossible to answer :D