Conservation of momentum question

[frasifrasi]

Question:
A shell is fired with an initial velocity of 20 m/s at an angle of 60° above the horizontal.
At the top of its trajectory, the shell explodes into two fragments of equal mass. One
fragment, whose speed immediately after the explosion is zero, falls vertically. How far
from the gun does the other fragment land? Assume that the terrain is level and air
resistance is negligible.

Should I do this with conservation of momentum for each dimmension? I don't know how I would get a distance from this...
So basically I am having a hard time getting the problem set up, I appreciate if anyone can help me with this.

 

[Doc Al]

Questions to guide you:
- Where is the top of the shell's trajectory? (Use coordinates where the gun is at the origin.)
- What is the shell's velocity and momentum just before it explodes?
- What is the initial momentum and velocity of the two fragments?
- Treating the "other" fragment as a projectile, where does it land?