Select the action you would use to solve x-3=12 . then select the property that justifies the action

The x is a placeholder for some currently unknown number. Let's say that number was 10. So we replace x with 10 to get 10-3 = 12. That statement is false because 10-3 is actually 7. What you can do is keep guessing until you get the right answer.

A much better approach is to undo what is happening to x. We have x and we subtract off 3 to get 12. Following this in reverse, we need to add 3 to both sides so that x is isolated.

x-3 = 12

x-3+3 = 12+3

x = 15

Let's replace x with 15 and see if the equation is true

x - 3 = 12

15 - 3 = 12

12 = 12

Sure enough we get the right answer.

The idea of adding 3 to both sides is possible through the additive property of equality (if a = b, then a+c = b+c)

In summary, I added 3 to both sides using the additive property of equality. Doing this isolated x to get x = 15.