Since the number of moles of a substance is the mass divided by the molar mass of the substance, we can just simply multiply the molar mass of magnesium chloride by the number of moles, which is 4.40 in this case.
To find the molar mass, refer to the periodic table for the relative atomic mass of Mg and Cl and add them together. Since there’s 2 chloride ions in MgCl2, double the relative atomic mass of Cl when adding.
24.31 + 35.45x2
=95.21
Now just multiply 95.21 to 4.40mol, which you’ll get 418.9g (corrected to 3 significant figures)