How do I find the mg/mL of a substance when given the molar mass if this sustance and the number of moles of it?

You also need the volume or the density, depending on the problem. You can't find concentration without some idea of the volume.

Molar mass (molecular weight) (grams/mole)

Number of moles (moles)

Density (grams/mL)

\frac {moles}{1} \times \frac {g}{moles} = g

If you have the volume (for example, it's a solution), you can divide by that

\frac {mass}{volume} = \frac {g}{mL}

If you have the density, you can calculate the volume occupied by the material:

\frac {g}{1} \times \frac {mL}{g} = mL

Since you now know the mass (g) and the volume (mL), you simply divide them to get g/mL.