Question 37228
If you want a point in which the x is opposite to y the point is (x,-x),
To find such point we have to solve the equation for y=-x

3x-y = 12, 
y=3x-12,
(-3)=3x-12,
-3+12=3x, 
then 9=3x, 
finally we get x=3
Therefore the point is (3,-3), In wich the x and the y are opposites.