Presumably, x is standing in for the question mark.
The pair of vertical angles (DFE and CFV) shown here both have measure 125°. This angle measure is equal to the average of the measures of the arcs intercepted by the chords CE and DV.
So we have
angle DFE = 1/2 (arc CV + arc DE)
125° = 1/2 (x + 200°)
250° = x + 200°
x = 50°
