2x-5y= -3
2x+5y=4
y=mx+b
-3 = 2x - 5y
-2x -2x
x = 5y
y=mx+b
4 = 2x+5y
-2x -2x
2x = 5y
Is this correct?
in no way
you cannot just throw away the -3 and the 4.
At any rate, if you plug your solutions into the original equations you will see that they do not work at all.
The real problem is that you are supposed to find a value for x and y which works in both equations. If you use the elimination method, just add the two equations together:
2x-5y= -3
2x+5y=4
-----------------
4x = 1
x = 1/4
Now, using x = 1/4 in either of the original equations, you get (using the first equation)
2(1/4) - 5y = -3
-5y = -7/2
y = 7/10
So, the pair (1/4,7/10) will satisfy either of the two original equations.