That’s an easy one. The SX has TRS (balanced) 6.3mm Jack out for booth. The Mackie TH15A have (balanced) XLR inputs. So you want to go from 6.3mm TRS Jack to MALE XlR. This one would do. I’d go for a 15 ft. rather have a few feet extra than struggle to place stuff where you want it because your 10ft cable isn’t quite long enough.
http://www.guitarcenter.com/Rapco-Horizon-XLR-TRS-Cable-336971-i1129192.gc
Depending on budget you can find cheaper (not sure I’d go for those) and (much) more expensive. Stay away from the gold-plated stuff. Not necessary and needlessly expensive. If you have the option, try to get cables that feature Neutrik connectors (best in the business). Those are worth paying a little extra for.
Don’t make too much out of it. The cable in the link will definitely do for your purpose, but I didn’t feel like going through their entire webshop :-).
Greetinx and good luck,
Chuck
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