Let's sum these two numbers.
9 + 21 = 30 total hours.
In every 12-hour the clock format changes AM to PM or the opposite PM to AM. So we need to divide 12 with the result 30.
There are 2 times 12 number in the 30
If the result is ODD number the time format will change the opposite side. If the result is EVEN number the time format will be the same.
Answer :6pm
What is 21 hours from 9pm ? = 6pm.
Add 21 hours to 9pm is equal 6pm.