It is not ideal, no. The RAW pin first goes into a 3.3V voltage regulator whose dropout voltage is up to 200mV (worst case). That means that you would get around 3.1V out, and it wouldn't be properly regulated.
Oh, it would work, but the power won't be stable, which means that any analogue readings that you make that don't directly use the (shakily) ...
The short answer is NO, you can't do that. Consider the size of the resistor needed to dissipate all that wattage and the fact that the current is not stable so it is practically impossible.
What you need is a DC to DC converter.
Is it as simple as hooking up a resistor in series with the Arduino to bring the voltage down to about 7V for the Arduino?
No, it certainly is not.
Can I make an assumption for the current to solve for the resistance?
No, you cannot. You never assume anything in electronics without a very very good reason.
You require a suitably rated voltage regulator (a ...