1) write the balanced equation:
4Al + 3O₂ ----> 2Al₂O₃
2) convert moles of Al₂O₃ to moles of Al using mole to mole ration
ration----> 4 mol Al = 2 mol Al₂O₃
3.5 mol Al₂O₃ (4 mol Al/ 2 mol Al₂O₃)= 7.0 mol Al
3) convert moles of Al to grams using its molar mass
molar mass of Al= 27.0 g/mol
7.0 mol Al (27.0 g/ 1 mol)= 189 grams