If we take the equation and multiply out the left side, we get....
+ 2xk + 
= + x +
The x^2 and k^2 cancel out on both sides leaving us with this...
2kx = x
If we divide both sides by 2x to isolate k, k ends up = 1/2
The x^2 and k^2 cancel out on both sides leaving us with this...
2kx = x
If we divide both sides by 2x to isolate k, k ends up = 1/2