Question 68747
x + y = 12
y = 2x 
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You are told that y=2x so substitute "2x" in place of "y"
in the 1st equation:
You get:  x+2x=12
3x=12
x=4
Now you know what "x" is.
Substitute that value of x into the statement "y=2x"
You get y=2(4)=8
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So the solution for the system of equations is (4,8)
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Cheers,
Stan H.