If the given measures are per unit volume and you mix equal volumes, then the result will be 9 mg per unit volume.
If the given measures are per unit weight and you mix equal weights, then the result will be 9 mg per unit weight.
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The resulting solution will have a concentration that is the weighted average of the constituent concentrations. when the weights (of the "weighted average") are equal, the resulting concentration is the ordinary average of the constituent concentrations. (12 +6)/2 = 9