So, I have an amplifier that runs off 12v 2amps DC, and I'd like to run it off 3 9-volt batteries. I'm not sure if I'll get enough power out of them by using a 7.5 ohm resistor, or if I'm doing my math wrong, or if it would even last long enough from the batteries. Help please.

Why the resister? And why 7.5 Ohm? Why do you need this?
Why not connect the (3) batteries and see how long it runs? The 12V, 2 amps you mention sounds like the rating of the amp, not necessarily the average power draw.