Vertical angles is the same angle, but we don't know "x". If you look in a way. We can do 180=(3x+80).
100=3x
100/3 = x
Now we have the answer for x we plug it in to get the answer.
3(100/3) + 80 =<D
<D = <C
Depends of the size of the quadrilateral in relation to the circle, but otherwise, yes
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