Lǎoshī zài hēibǎn shàng huà le yí ge píngjiǎo lái yǎnshì.
The teacher drew a straight angle on the blackboard to demonstrate.
Tips
memory
平 means flat or level, and 角 means angle. A 平角 is literally a 'flat angle' - it looks like a perfectly flat, level line, which is exactly what a 180° angle looks like.
