Left belt is alternator/Power steering. Right belt is A/C
If its a stretch fit A/C belt like in the photo, there is no adjustment. So can't sort that.
But, loosen both alternator mounting bolts, then wind the bolt on top of the P/S bracket clockwise, tighten's thet alt belt.
You should just about be able to turn the belt sideways on itself at the longest part. DO not over tighten it, it will damage the alternator bearings.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i