Yeah, I think you need to work out the volume from mass and density then the buoyancy is equal to the weight of water displaced (I think, been a looooooong time since I did this stuff!). You've then got gravity pulling the iron down and buoyancy pushing it up giving the apparent weight of the iron.
That's what I'd be doing, anyway.
Edit: for shapes and stuff you would usually do length x breadth x height or whatever but if given any two of mass, volume and density you can work out the third since they are simply related.