Variable Capacitor?

The diagram below depicts a variable capacitor, such as those that are used in radios. The knob is rotated so that the effective area of the plates can be altered, thus causing variable capacitance. For one such variable capacitor, the capacitance varies from 6.35 × 10-4 F to 6.40 × 10-5 F. The capacitor is charged to a potential difference of 210 V at maximum capacitance and then isolated. At minimum capacitance, what is the voltage?