Can someone justify this answer

Keysha, are you looking for a solution / answer? Right now it's just an equation, so nothing to justify yet...?

You need to move the x's to one side and the numbers to the other. There are always several ways to do this.

Here's one:

13x - 18 = -5x + 36 [now add 18 on both sides]

13x = -5x + 54 [now add 5x on both sides]

18x = 54 [now divide by 18]

x = 3