How to stop the dc motor using mq3 sensor

hi everyone!!

I'm doing project by using arduino..

High sensitive alcohol sensor with auto car ignition disable

I wrote code to detect alcohol by using sensor MQ3 but now i got problem how to stop the dc motor when the alcohol detect and motor should run when alcohol value will not detected

please help me..

 if (alcohol_level > limit) stop_motor();
   else start_motor();