The lines through the sides in the circle indicate that the three sides are all of the same length, and therefore share the same angle.
A shape inside a circle will always have its angles equal 360, so you can find the individual angle with the following equation:
[tex]120 + 3x = 360[/tex]
Subtract 120 from both sides.
[tex]3x = 240[/tex]
Divide both sides by 3.
[tex]x = 80[/tex]
mBC's angle is 80 degrees.
