Since what's inside the absolute value sign can be positive or negative, we have two equations.
5x - 12 = x
4x = 12
x = 3
5x - 12 = -x
6x = 12
x = 2
Let's check.
|5(3) - 12| = 3
|15 - 12| = 3
|3| = 3
3 = 3
0 = 0
True
|5(2) - 12| = 2
|10 - 12| = 2
|-2| = 2
2 = 2
0 = 0
Also true