What potential difference is needed to give a helium nucleus (q=2e) 85.0 kev of kinetic energy?

The kinetic energy K given to the helium nucleus is equal to its potential energy, which is 

where q=2e is the charge of the helium nucleus, and  is the potential difference applied to it.
Since we know the kinetic energy, we have

and from this we can find the potential difference:

The answer is a 0.64 volts