Possible calculation error using current divider rule on a non ideal current source circuit
It's identical to the current divider rule shown in Wikipedia: 



Rt = 3K here (the resistor in parallel with the current source)
Rx = 6K
It = 9mA 

So \$ I_X = I_T \cdot \frac{R_T}{R_X+R_T} = 9ma\cdot \frac{3K}{3K + 6K} = 3mA\$