Unfortunately, the voltages don't work out in your favor. If you were using a motor that ran off 7 to 12V you could power the motor and the arduino using the arduino exteranl dc power jack wired in parallel with the motor.. B6 ohm 2W resistor ecause the motor you are using runs off 6 volts, the only way to power them both is to use a 9V AA battery pack add a series dropping resistor for the small dc motor. The 9V plugs into the ext.dc power jack of the arduino and the resistor is wired in series with the small dc motor and the 9V battery pack. In order to do this , you would need to know how much current that motor draws at 6v by putting a meter in series (on 10A current scale, plugged into the 10A jack of the meter ) . Using an example that assumes you measure 0.5A (500mA) at 6V, and Vload =6V, the dropping resistor value would be (Vin -Vload)/Iload=(9V-6V)/0.5A=6 ohms. The power rating of the dropping resistor would be P= Iload x Vdropped=0.5A x 3V =1.5W, so you would use a 2W , 6 ohm resistor in series with the small dc motor. If the current measured was 1A, then Rdropping=9V-6V/1A= 3 ohm, and the power rating would be P= 1A x 3V= 3W.