atomic mass number question

sorry, i'm having trouble grasping a lot of this... i feel like i have a general understanding of what's going on but a lot of the problems are still confusing to me...

this problem

An element in its solid phase has mass density 1750 kg/m3 and number density 4.39 * 10^28 atoms/m3.

asks what is the element's atomic mass number

i thought that it should just be 4.39*10^28 divided by 1.661 *10^-27 = u, but then i'm not sure where the mass in kg comes in to play
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 1 au = 1.661 *10^-27 Kg.
 yeah, i know the atomic unit number but how does it relate to mass density?

atomic mass number question

can anyone point me in the right direction? i'm still not getting this
 Recognitions: Gold Member Homework Help Science Advisor $$\rho=nm$$ where rho is the mass density, n is the number density and m is the mass of the atoms.