Question 246615
Y = 2X + 9
2X - Y = -9 USE SUBSITUTION

Since Y = 2X +9 in the first equation substitute 2X+9 for Y in the second equation:

2X-Y = -9
2X-(2x+9) = -9
2X-2X-9 = -9
-9 = -9

So all values of X and, consequently, Y are solutions to these two equations.