Capacitors and Dielectrics of a circuit

[spoonthrower]

Two capacitors are identical, except that one is empty and the other is filled with a dielectric ( k= 5.00). The empty capacitor is connected to a 6.0 V battery. What must be the potential difference across the plates of the capacitor filled with a dielectric such that it stores the same amount of electrical energy as the empty capacitor?

So here are my thoughts so far...
V= 6 V
Energy=.5CV^2
So the energy of the empty capacitor is .5V^2 which is .5(18^2)=18 right? they give no value for C so i assume it is one?
I have no idea where k comes into the picture. please help. thanks.

 

[berkeman]

What is the equation for the capacitance of a parallel plate capacitor in terms of epsilon, A (area) and d (separation distance), ignoring fringe effects at the edges? What is the relationship between epsilon-zero (vacuum value) and the epsilon inside the dielectric?

 

[spoonthrower]

Answer

I just figured it out.
Energy=.5CV^2
Energy=.5kV^2

so .5CV^2=.5kV^2
so CV^2=kV^2
C=1
so V^2=kV^2
so 6^2=5V^2
so V= sqrt(36/5)=2.7 V