Yes, rms voltage = square root of (power x resistance), that MAY be what they are looking for.
Problem is that "voltage" and "potential" are used interchangeably, so saying "potential voltage" is like saying "voltage voltage". If they mean PEAK voltage, then it's 28 V.
So, the potential voltage is 20 volts rms across the load.
for calculating the rigt speaker we need a 20 volts of voltage potential in an amplifier , am i right?
how about if we are going to multiply Erms to 1.414? what does it mean again?
Are you talking about the positive rail (supply voltage) of the amplifier? For that you need MORE than 28V. Like Hero999 said, the DC supply minus the saturation drop on the drivers.
of coure when calculating a right speaker we need to find out first the source voltage of amplifier which can deliver a voltage potential, some say that it easier to measure the potential voltage across the load.
so, what is a voltage potential? is it in Vrms or Vpeak?
Do you have a scope? Clip one channel to one side of the resistor. Clip the other channel to the other side. Ground the scope to the amplifier ground. Select "Add". Select "Invert". This will subtract channel 1 from channel 2 and give you the difference in voltage.