Question 113959
4X+2Y=10
2Y=-4X+10
Y=-4X/2+10/2
Y=-2X+5
{{{ graph( 300, 200, -6, 5, -10, 10, y = -2x +5) }}} (graph 300x200 pixels, x from -6 to 5, y from -10 to 10, y = -2x +5). 
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3X-Y=-7
X+Y=-9
----------- NOW ADD.
4X=-16
X=16/-4
X=-4 ANSWER.
3*-4-Y=-7
-12-Y=-7
-Y=-7+12
-Y=5
Y=-5 ANSWER.
PROOF
-4+(-5)=-9
-4-5=-9
-9=-9