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Explanation:
Refer to the diagram below. I've added the variables b, c and d to the drawing you posted.
The variable b is an inscribed angle, while variables c and d are arc measures.
For any inscribed quadrilateral, the opposite angles always add to 180. They are supplementary angles. This means the 110 degree angle and the angle b add to 180, so,
110+b = 180
b = 180-110
b = 70
Inscribed angle b is 70 degrees.
Next, we apply the inscribed angle theorem. This theorem says that the inscribed angle doubles in value to get its corresponding central angle measure (and therefore it's arc measure as well).
In other words, inscribed angle b = 70 doubles to 2*70 = 140 degrees. This represents the combination of arcs c and d. Therefore, c+d = 140. Note how inscribed angle b cuts off this longer arc.
We don't need to find the measure of arc c or arc d individually. We just need to find the measure of arc 'a'.
The four arc measures (161, a, c and d) all combine to a full 360 degree circle. Let's add those four parts, and set that equal to 360. We'll keep in mind that c+d = 140 as well.
So...
a+c+d+161 = 360
a = 360-161 - (c+d)
a = 360-161 - (140) ..... replaced "c+d" with "140"
a = 59
Arc 'a' is 59 degrees
