Although this is probably not what your professor wants, you can see the theory and calculations on using a resistor network to shift almost any input voltage range into the 0-5V (or any other range) in my design idea
"Use Excel To Calculate A-D Level-Shifter Resistor Values"

http://www.elecdesign.com/Articles/...leID=12123

As others have said it is possible to use an op-amp to achieve this, and this is often preferable because of impedance considerations.

With a +/-2 (AC volts peak to peak I presume), you could easily shift using a series capacitor followed by a resistive divider of equal resistor values. Your input would then go from 0.5V to 4.5V with the loss of some of the dynamic range.

All of this is for instantaneous readings. If you are going into RMS then all this is simply a precursor to the mathematical process others have alluded to.

-Aubrey