How many atoms of Aluminum are contained in this compound?

This means that the mass of one mole of aluminium atoms will be 26.982 g. As you know, one mole of any element contains exactly 6.022⋅1023 atoms of that element. This means that if you know how many moles of aluminium you have in 4.55 g, you can use Avogadro's number to find the number of atoms.