Just a friendly reminder, everyone knows Ohm's so please remember it is just V=IR for a resistive load, and V=IZ for a reactive load.
Z is a phasor, or incorrectly called, vector, that has the magnitude and phase components.
To calculate power, you can use the power formula P = V x I x Cosine of the phase angle between V and I (the voltage and current phasor)
For a resistive load, the phase angle is virtually 0, and cosine 0 degree is 1
For a reactive load such as the impedance of a loudspeaker, the phase angle can be as high as 60 degree, and cosine 60 degrees = 0.5 so you can do the math and figure out how much watts would end up in the amp instead of the speakers.
You can also see that the phase angle does not factor into the current (I) calculation using Ohm's law, but it obviously affect the power calculations.
ADTG: I hope you aced in trigonometry too.