Apparently, the 2 g of antibiotic occupies 1.4 ml of volume. That would make the final volume 10 ml after adding 8.6 ml of water. When asking for the concentration, there are several ways of expressing concentration. It can be given in g/ml, g/100ml, g/liter, etc. or it could be stated in molarity, for example. For the latter, you'd need to know the molar mass of the antibiotic. Since that's not given, you are stuck with stating the concentration as g/ml. That being the case, the concentration would be correctly stated as 0.2 g/ml (2g/10ml = 0.2g/ml).