x + y= 9 and x - y= 3
rearrange the second equation:
x - y= 3
x = 3 + y
x -3 = y
substitute y with "x-3" in the first equation and solve
x + y= 9
x + (x-3)= 9
2x -3 = 9
2x = 12
x = 6
you could also start with the first equation and substitute y in the second.
graphically speaking, it's the intersection point of to lines, at least the six-part of the point.
