Actually you don't need . We don't even need the reaction because we don't even consider it.
You want the molarity (i.e. # of moles divided by volume) of chromium (III) nitrate. We're given that they're 9.3 grams of it (which can be converted into moles) and we're given that it's put into a 30 mL solution.
So to find its molarity ... ?