A6.0-v battery maintains the electrical potential difference between two parallel metal plates separated by 1.0 mm. what is the electric field between the plates?

The voltage difference between the two plates can be expressed in terms of the work done on a positive test charge q when it moves from the positive to the negative plate.
E=V/d
where V is the voltage and d is the distance between the plates.
 
So,

E=6.0V/1mm= 6000 V/m. The electric field between the plates is 6000 V/m.