Those red and white outputs on the back of the Scarlett Solo are called RCA jacks. In the case of the Scarlett, those jacks are outputs. You can connect them to any RCA inputs using an RCA cable.
This is what an RCA cable looks like:
Get an RCA cable and connect it to the outputs of the Scalett, matching the red plug to the red jack and the white plug to the white jack. Plug the other end of the cable (white to white and red to red) into the left side of the group of four RCA jacks on the Mackie. That's the side the says "Tape In" to the left of the jacks.
Now look at the main fader, which is the one farthest to the right. Right above the main fader is a knob labelled "Tape Level". Play sound from your computer and turn the "Tape Level" knob slowly to the right until you hear the volume you want.
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