Can a 6 ohm sub work for computer

In matching a speaker to a source (amp) you have to consider both the impedance and the power ratings. Many larger older speakers (especially subwoofers) were designed to put out lots of sound by using lots of input power from the amp. But the average PC sound card does not have a big powerful output amp stage. So although the speaker may work, you may not get enough sound out of it. That is, unless you also can provide a small amplifier between PC and speaker. The amp could be a bit tricky, too, because most amps are NOT intended to be fed from a speaker output designed for a 4-, 6- or 8-ohm load.