in circle o, ac and bd are diameters.
what is mbc?
50°
80°
100°
130°

130

step-by-step explanation:

In the circle, the angles boa and doc are vertical angles, on the opposite side of the intersection, and thus equals

that means that whatever 3x - 70 is, equals x + 10

so,
3x - 70 = x + 10
2x = 80
x = 40

so.. hmm if x = 40, then arcba is 3(40) - 70, or 120 - 70, which is 50

if arcba is 50°, then her twin arccd is also 50°, add them both and you get 100°

well, a circle has a total of 360° only, if we subtract those twins, we get leftover with 260°

now, because the other two angles, boc and aod are also vertical, they're also equals, and so that makes arcbc a twin of arcad

if we get the leftover 260° and divide it evenly to the twins, they each get 260/2 or 130°