Question 929719
x = 2y-9
 -2x+4y = 18 
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Substitute for "x" and solve for "y"::
-2(2y-9) + 4y = 18
-4y + 18 + 4y = 18
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18 = 18
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That is always true and tell you the two equations
are really the same equation.  
The solution is all points of the form (x,(x+9)/2)
where x is any Real Number.
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Cheers,
Stan H.
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