The simplest way to do it is to use Thevenin's Theorm.
The Thevenin voltage is 6 Volt and the Thevenin resistance is 500 Ohm (the two 1k resistors in parallel).
So a 12V source with a 1k, 1k voltage divider is equivalent to a 6 Volt source in series with a 500 Ohm resistor.
But you need to know what current the motor draws when it is running. The static resistance is only of use if you want to calculate the starting current.
If the motor is 8 Ohm, then the starting current will be 6/(500 + 8) = 6/508 = 11.8 mA.
The starting voltage will be 8 * 0.0118 = 0.0994 Volt.
So it is VERY unlikely that the motor will start!!
What you need is a 6 Volt voltage regulator that will maintain the voltage at about 6 Volt regardless of the current drawn.
The starting current will be 6/8 = 0.75 Amp. So you need a regulator that can dissipate at least 6 * 0.75 = 4.5 Watt.