9y=-3x-15 x+3y=-5 solve for systems of equations

First equation, divide both sides by 3
3y=-x-5
what a coincidence
3y=-x-5
sub into other equation

x+(-x-5)=-5
x-x-5=-5
0-5=-5
-5=-5
true
there are infinite solutions
they are the opints that lie on the line 9y=-3x-15

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