Irene R. answered 04/23/20
BS in Mechanical Engineering and Certified math teacher for 13 years
I assume you need to write an equation to represent this sentence.
If we use the variable x to represent the unknown number, then "twice a number" would be written as: 2x
Then "nine more" would be written as: 2x + 9
Finally, "is 21" would be written as: 2x + 9 = 21