Explanation:
x+3y=12--(1)
x=-6y--(2)
put (2) in (1)
-6y+3y=12
-3y=12
divide both sides by -3
y=(12/-3)
y=-4
put y=-4 in (1)
x+3(-4)=12
x-12=12
x=12+12
x=24
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