Falling Rock Problem

1. The problem statement, all variables and given/known data

A 500g rock is thrown straight down from a bridge and hits the water 5.2 m below. If the rock strikes the water at a speed of 12.5 m/s, what was the initial speed of the rock?

2. Relevant equations

I'm not totally sure...

3. The attempt at a solution

I have no idea how to even start this. Could someone help me start to solve it, please?

 

It's apparently supposed to be a momentum equation...but I have to disagree with them there...

Well, I suppose vf^2 = vi^2 + 2ad is worth a shot.

Which gives me...

Vi^2 = vf^2 - 2ad

That works out to the right answer...but it doesn't look like it belongs on a momentum assignment.

Thanks!