No computer has left and right sub outputs.
If your PC has a sub output it will be a mono one combined with the center channel output on the orange audio jack. If the sub has two inputs use the one marked mono or LFE or either left or right if they aren't marked.
If you PC has just left and right full range outputs then you split them so you can connect both the sats and sub. Not as good since the sats are gettting a full range signal but works OK.