f(x) = 2x + 6/x I know its not a one to one function but how would I prove it?

y = 2x + (6/x)
Solve for x:
xy = 2x^2 + 6
2x^2 - yx + 6 = 0
x = (-(-y) +/- sqrt((-y)^2 - 4*2*6)) / (2*2)
x = (y +/- sqrt(y^2 - 48)) / 4
Since this is a quadratic, as long as y is real and the discriminant is positive, i.e. y^2 - 48 > 0, then there are two solutions for x. Therefore the function is not one-to-one.