Energy Stored leading to Dielectric breakdown

 P: 33 1. The problem statement, all variables and given/known data A cavity can only store 1J of EM energy as it reaches dielectric breakdown at 3kV/mm. What dielectric strength would a new gas filling the cavity have if the energy stored was to be 4J. 2. Relevant equations U=$$\frac{1}{2}$$(epsilon)E2 3. The attempt at a solution I would use the above equation to find the E field that would result in 4J of energy stored. Can anyone tell me if this is the correct approach to answering this question?