Since there is no picture, I cannot give you a specific answer, but...
To decide if a line best fits a scatterplot you have to look at whether or not the line even remotely makes sense, if the dots are on one side and the line is no where near any of the dots, it is probably not a realistic placement.
If it looks realistic, take a look at if it best fits the dots, is there a line that would touch more or be nearer to more of the dots?