It almost never proves beneficial for this sort of third-person tutoring.
One very important thing to observe, when solving, graphing, manipulating, or just playing around, is how to make your life easier. This is what pka showed you. It was a massive step toward just what you are asking. If you see it as only a clue in the wrong direction, this conversation is not going to go well untill you upgrade some other skills.
The "rule" is to proceed in a rational and logical fashion.
In this case, there is simplicity.
Simply
1) Solve 0.6x - 3 = 0.6
2) Check which side of the solution "works".
3) Decide if you should included the end point.