The voltage given is the Max Voltage.
First find the total resistance, which is simply
R total = R1 + R2 (*where R1 = 8.6 , R2 (speaker) = 10.4 )
Next calculate the Max Current.
Max Current = Max Voltage / R total
Now convert this to Rms Current:
Rms Current = Max Current / √2
Now you can calculate the Power in the speaker:
Power = Irms^2 * R2 (*where Irms = Rms Current)