Heater cartridge with 7.2 ohms - 12 or 24 V?
e3D Heater Cartridges are documented to be around 4.8 Ω for 12 V &amp; 30 W, 3.6 Ω for 12 V &amp; 40 W, 19.2 Ω for 24 V 30 W and 14.4 Ω for 24 V 40 W.

7.2 Ω is a value quite far away from these values - about double of what the 12 V versions are listed and about a third/half of a 24 V heater cartridge. So it is not a cartridge that is similar to those. I have contacted e3D about updating the specs of their High Precision Heater Cartridges to include the nominal resistances to get more references, but that information is pending. 

However, we don't need that. A variant of Ohm's law can help us estimate what this cartridge could be: Power is the square of Voltage (U) divided by Resistance.

$P=\frac {U^2} R = \frac {144\ \text V^2}{7.2\ \Omega} = 20\ \text W$

$P=\frac {U^2} R = \frac {576\ \text V^2}{7.2\ \Omega} = 80\ \text W$

Mounted in a 12 V Machine this should act work at 20 W, in a 24 V it would work at 80 W. It does not say, however, if it is designed to get powered with 12 V or 24 V. It doesn't help, that both types exist: I was able to find 12 V 20 W heater cartridges just as well as 24 V 80 W heater cartridges. If it is an underpowered 20 W one and mounted in a 24 V machine, you risk burning it though, but not the other way round.

To differentiate what exactly you have, a photo of the heater cartridge would be needed - it should bear at least a marking of what it's nominal wattage or voltage is.