To find the number of atoms in 238.4 g (Al), we can convert the mass into number of moles, using molar mass, (found on a standard IUPAC periodic table) and thus, multiply this result by Avogadro's Constant, 6.022×10²³, i.e. the number of 'items' in a mole (atoms, molecules, ions, etc...).
n(Al) = mass present (g) ÷ molar mass of aluminium (g/mol)
n(Al) = m/M = 238.4/26.98 = 8.836 mol (4 sig. fig.)
N(Al) = number of moles × Avogadro's Constant
∴ N(Al) = 8.836 × 6.022×10²³ = 5.321×10²⁴ atoms of aluminium