If x + 3y = Kx ---> K = (x + 3y)/x [Assuming that x is not equal to zero.]
If y + 3x = Ky ---> K = (y + 3x)/y [Assuming that y is not equal to zero.]
Set these equations equal to each other: (x + 3y)/x = (y + 3x)/y
Cross multiply: xy + 3y2 = xy + 3x2
Simplify: 3y2 = 3x2
Simplify: y2 = x2
Re-arrange: y2 - x2 = 0
Factor: (y - x)(y + x) = 0
Therefore: y = x or y = -x
If y = x: x + 3y = Kx ---> x + 3x = Kx ---> 4x = Kx ---> K = 4
If y = -x: x + 3y = Kx ---> x + 3(-x) = Kx ---> -2x = Kx ---> K = -2