Question 308437
If these are linear equations then:
{2x+y=5
y=-2x=5 (red line)
{4x+2y=6
2y=-4x+6
y=-4x/2+6/2
y=-2x+3 (green line)
{{{ graph( 300, 200, -6, 5, -10, 10, -2x +5, -2x +3) }}} (graph 300x200 pixels, x from -6 to 5, y from -10 to 10, of TWO functions -2x +5 and -2x +3). 
These lines are parallel thus no unique solution for x & y.