Reply to audio stereo setup

I'm having trouble figuring out how to set up my system at home. This is how it is currently set up.

-Headphones are in headphone jack
-Computer speakers plugged into Main out 1 using a 3.5mm/1/4" adapter

Preferences: Output master: 1: Out: Master Left and 2: Out: Master Right

My problem is I have sound coming out of my headphones and only the left speaker.

Do I need a different adapter so that the 3.5mm plug coming out of my speakers are going into out 1 & 2? Or is there a way to do this with my current set up