If a 325 watt heater has a current of 6.0A what is the resistance?

Watts = Current x Volts with your resistive heat application. To figure out resistance you need to know voltage and current. Since you are drawing 6 amps then Volts = 325/6. This means that there is about 54 volts supplying the heater which seems like a very strange supply voltage. Since Volts = Current x Resistance the resistance = 325/36.