You need to work backwards with this one..I will use your numbers, but you can figure this out for any combination you want…. Your numbers: 7.4 volts, 9 watts, 5 ohm atty.
To get 9 watts out of a 5 ohm atty you need to pass 1.34164 amps across it.
I = Square Root of (P / R).
To get 1.34...