Answer:
x=30
Explanation:
You have to isolate for x. This means you need to somehow get x on one side of the equation all alone. In this case it is x-9=21 which means some unknown number x with 9 subtracted from it is equal to 21. When we bring the -9 over to the other side to get the x all alone, we have to switch the sign of the -9 so it becomes +9. This means x=21+9, and if you add that you'll get 30!