We have a quadrilateral inscribed in a circle.
The quadrilateral is a trapezoid.
When we have a quadrilateral inscribed in a circle, opposite angles are supplementary.
Then, we can write:
[tex]\begin{gathered} m\angle C+m\angle B=180\degree \\ 4x+(4x+11)=180 \\ 8x=180-11 \\ 8x=169 \\ x=\frac{169}{8} \\ x=21.125 \\ x\approx21.1 \end{gathered}[/tex]Answer: x = 21.1