I saw someone solve this problem using: ? g v = mg

for the problem: a barge with vertical sides 6 m wide and 10 m long is floating.how much deeper does it sink with 15,000 kg of coal added to it.

I've never seen the equation they used, the equation my teacher gave is
?obj V obj = ? fl Vfl

and I don't know how these two equations are related and why its so hard to use the equation my teacher gave to me.
thanks for the help!