Whith graph shows a system of equations with a solution at (2,-1)?

The last graph.

Explanation:
Since you are looking for a solution you need to look at the place where both lines connect, in this case, we are looking for the coordinate ( 2, -1 ) 
2 being the x-axis and -1 being the y-axis
The last graph is the only one that shows that the two lines touch at the coordinate ( 2, -1 )