Answer:
BC = 100°
Explanation:
You want the measure of arc BC formed by two chords that intersect at 85° and intercept an opposite arc of 70°.
Angle at crossing chords
The angle formed by crossing chords is the average of the angles they intercept. Here, that means ...
85° = (? +70°)/2 . . . . the crossing angle is 85°
170° = ? +70° . . . . . . multiply by 2
100° = ? . . . . . . . . . subtract 70°
The intercepted arc BC is 100°.