Answer:
124°
Explanation:
You want the measure of the angle marked (4+10x) where chords cross. The chords intercept arcs marked (9x+20) and (10x).
Angle relation
The measure of the angle where chords cross is the average of the measures of the intercepted arcs.
((9x +20) +(10x))/2 = 4 +10x
19x +20 = 20x +8 . . . . . . . . . multiply by 2
12 = x . . . . . . . . . . . . . . subtract (19x+8)
The angle at E is ...
4 +10(12) = 124
The measure of angle DEC is 124°.
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Additional comment
Arc DC is 128°; arc BU is 120°.