Calculate resistance of an unknown resistor in a circuit
I seem to have gotten the right answer now, what I did was that I used KCL as follows:

8mA - 2mA(From the unknown resistor) - ( 0 - Vu / R3) = 0

I then moved the equation around as following:

-8mA + 2mA = Vu * ( - 1/R3)

Then:

-6mA / (- 1/R3) = Vu = 12 V

hen, by using Ohm's law I calculated the resistance:

R = V / I = 12 / 2mA = 6000 Ohm's