Momentum Conservation Questions

An explosion blows a rock into three parts. Two parts go off at right angles to each other: a $1\text{kg}$ part moving at $12{\text{ms}}^{-1}$ and a $2\text{kg}$ part moving at $8{\text{ms}}^{-1}$. If the third part flies off with a velocity of $4{\text{ms}}^{-1}$, its mass would be: