Question

A uniform, solid, spherical asteroid with mass 1.2 * 10^13 kg and radius 1.0 km is rotating with a period of 4.6 hours. A meteoroid moving in the asteroid's equatorial plane crashes into the equator at 9.0 km/s. It hits at a 58° angle to the vertical and embeds itself at the surface. After the impact the asteroid is rotating in the same direction with a period of 3.0 hours. Find the meteoroid's mass.

I know this has to do with angular momentum, but I'm really lost on how to go about this >.< please help.