Molar volume is when 1 mol of any gas occupies 22.4 L at STP.
Methyl ether has a mass of 8.12 g,
Volume occupied - 3.96 L
If 22.4 L occupied by 1 mol of gas
Then 3.96 L occupied by 1/22.4 x 3.96 = 0.176 mol of gas
The mass of 0.176 mol = 8.12 g
Molar mass is mass of 1 mol
Therefore mass of 1 mol = 8.12/0.176 = 46.1
Molecular weight is 46.1 g/mol