Solve the system of equations by graphing
x+y=8 x-y=-4

To solve it by graphing just means to graph both lines.  The solution to the system of equations will be the point where the two lines intersect.  So if you use google you can type in "y=8-x, y=x+4" and it will graph both lines.  What you will see though is just two parallel lines which never intersect because both lines have a slope of 1 and different y-intercepts.

There is no solution to the system of equations

4should be the last one n=4