Hi @vibrantrobin !

When you use \(\frac{a-b}{2}\), remember that \(a\) is the arc opposite of the angle on the other side of the circle, while \(b\) is the arc close to the circle.

Here's a visual of what the formula looks like:

Do you see how, in the original problem, the larger arc is \(\frown \atop AB \) \( = 130^{\circ} \) while the smaller arc is the \(30^{\circ}\) arc. Then we can just do \(\frac{130^{\circ}-30^{\circ}}{2} = 50^{\circ}\)