LM1877 2.1 CH Amp/ Interace with Summing Amp

I need to design a 2.1 ch amp using the LM1877. I understand how to get my values for the left and right channels but I am required to use a summing amp to interface both the left and right channels to the inputs of the summing amp and then construct a low pass filter on the output of the summing amp to then therotically drive a subwoofer that is designed to respond to lower frequency's. No filtering is required on the output of the left and right channels.

You can make a simple summing circuit with 2 resistors or make an audio mixer with an opamp.
You can make a simple lowpass filter with one resistor and one capacitor but it won't do much or you can make an active 2nd-order or 3rd-order lowpass filter with an opamp and some resistors and capacitors that works good.

Usually when there is a sub-woofer the stereo amplifiers are fed from highpass filters.

Like you or someone else in your class was told in one of these forums, the LM1877 is a very low power stereo amplifier IC. You will need a more powerful mono amplifier to drive a sub-woofer.