The solution of the system of equations means that following:
1- In case we are dealing with system of equations, the solution would be the point of intersection of the two graphs
2- In case we're dealing with only one equation, the solution would be the points of intersection of the graph with the x-axis
Graph (a):
The graph is for a system of equations (two lines). Therefore, the solution would be the point of intersection of the two lines.
From the graph, we can note that there is only one point of intersection between the two lines (-3,-1). Therefore, the system of equations has only one solution
Graph (b):
The graph is for a system of equations (two lines). Therefore, the solution would be the point of intersection of the two lines.
From the graph, we can note that the two lines are parallel. This means that they will never intersect. Therefore, the system of equations has no solutions
Graph (c):
The graph is for a single line. Therefore, the solution(s) would be the point(s) that make the overall equation equal to zero, i.e. point(s) of intersection with the x-axis.
From the graph, we can note that the line intersects with the x-axis only once at point (2,0). This means that the line has only one solution.
Hope this helps :)