Step-by-step explanation: Discussion What you have presented is called a cyclic quadrilateral. A cyclic quadrilateral is a 4 sided figure whose angles (4 of them) all touch the circumference of a circle.
Here's what you want to know. Two angles that are opposite each other are supplementary (they add to 180 degrees)
Equation x + 80 = 180 Subtract 80 from both sides.
