In order to answer this, we need to first find out which reactant is in limiting supply. That reactant will dictate how much product can be formed.
For Mg: 6.9 g Mg x 1 mol Mg/24.3 g = 0.284 mol Mg x 2 mol MgO/2 mol Mg = 0.283mol MgO
For O2: 450 g O2 x 1 mol O2/32 g = 14.1 mol O2 x 2 mol MgO / 1 mol O2 = 28.2 mol MgO
Clearly the Mg is in limiting supply, so only 0.283 moles of MgO can be produced
The mass of MgO is calculated as...
0.283 mol MgO x 40.3 g/mol = 11.4 g MgO can theoretically be formed