First we interface both the motor of car with motor driver IC L293D as shown in the following diagram:

In the previous diagram:

• OUT1 (pin 3) has been connected to one terminal of LRM
• OUT2 (pin 6) has been connected to second terminal of LRM
• OUT3 (pin 11) has been connected to one terminal of RRM
• OUT4 (pin 14) has been connected to second terminal of RRM
• VS is connected to positive terminal of 9V DC battery
• GND is connected to negative terminal of 9V DC battery
  

The OUT pins provide DC output voltage to drive the motor. The value of voltage output of out pins is decided by the voltage applied to pin VS of L293D. In our circuit diagram we have used a 9V DC battery which will power our 9V motors (Note that this 9V battery is not used to control the speed of motor). Make sure you choose your power source precisely as per rating of motors. If a very high power source is used it can damage your motor and if
underpowered, it will not drive the motor at all.

In case you don't want to use batteries, DC adaptor is a good replacement which will supply the required voltage output as per requirement.