Looks like stoichiometry.
Magnesium oxide = MgO
Gotta know the equation for how it is formed: Mg + O2 ⇒ MgO
Then you have to balance the equation. 2Mg + O2 ⇒ 2MgO
Look on the periodic table to find molar mass of Mg and O
Mm Mg = 24.31
Mm O = 15.999
The equation shows you have 2 mol Mg makes 2 mol MgO
So molar mass of 1 mole MgO = 24.31+15.999 = 40.309
We need 2 moles of MgO because we had 2 mol Mg so
2 mol MgO x 40.309 g = 80.6 g