y=13-3x

2x+9y=-8



the first equation tells you that y=13-3x.  so, replace y in the second equation with (13-3x).



2x+9(13-3x)=-8.   



simple enough.  Now solve.[distribute]



2x+117-27x=-8  -117, both sides

-25x=-125  divide by -25

x=5 

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