A solution is where the lines intersect. Graph A shows exactly one solution, and it is at (-2,2)
Graph B doesn't have any solutions since the lines are parallel, and parallel lines never intersect. We consider this system to be inconsistent.
Graph C has infinitely many solutions. The two lines perfectly overlap, meaning there are infinitely many intersection points. All solutions here are of the form (x,y) = (x,2x-1)
