Here AC is the median of the isoceles trapezoid DEBF. Formula to find the median of a isoceles trapezoid is:
AC=

Next step is to plug in the values from the diagram in the abov formula. So,

Multiply by 2 to each sides.
11= x+4.4 Simplify.
11-4.4= x Subtract 4.4 from each sides.
So, x=6.6