Assuming that you're in the far field (and in a vacuum or air), then you may assume that the freespace impedance is 377 ohms (120*Pi ohms). The rest is just Ohm's law and simple math. Pay attention to the units of distance and area. Bring the units along with the numbers as a sanity check. It should be volts per meter (linear) and watts per square meter (area) - for example.
