The density measurement can be used as a conversion. It allows you to go between grams and mL. When you say the density of mercury is 13.6 g/mL, you are technically saying that there are 13.6 grams of mercury per every 1 mL. So, if you want to convert between grams and mL, you need to multiply the staring amount by this conversion. Because you want to cancel out grams, you need to put this unit in the denominator of the ratio.
455 grams mercury 1 mL ------------------------------ x -------------------- = 33.5 mL mercury 13.6 grams
