If the lines are parallel (have the same slope but different intercepts) . . . no solution, because they never cross.
If the lines have different slopes . . . one solution, because they have exactly one common point.
If the lines have the same slope and the same intercept . . . infinitely many solutions, because every point on one line is also a point on the other line, one lays right on top of the other, and when you look at them on the graph, it looks like only one line.
