We must consider the relation between Al and [tex]O_2[/tex]:
[tex]\frac{3\ mol\ O_2}{4\ mol\ Al} = \frac{4,5\ mol\ O_2}{x}\ \to\ x = \frac{4,5\ mol\ O_2\cdot 4\ mol\ Al}{3\ mol\ O_2} = \bf 6\ mol\ Al[/tex]
As you can see, the number of moles of Al is double to the number of moles of [tex]Al_2O_3[/tex], keeping constant the proportion of both substances.