First we have to find the Molar Mass of Magnesium Chloride (MgCl2)
Molar mass of
Mg-24.30
Cl-35.45x2 (we do times 2 because there are 2 Cl atoms)
Add the molar masses together, and you get 95.20
One mole=95.205g
And there are 3.90 moles so
95.205×3.90=371.28
3.90moles of MgCl2= 371.28g