Could someone show me how to solve system of equations by graphing?
9x-9y=-45
9y-9x=45
Those two equations are really the same except for a sign change on both sides. They are not independent. Both will plot as the same line on a graph.
Since you have only one line instead of two intersecting lines, there is no single solution. Any x,y combination with y-x=5 is a solution.