$\text{ Mass of }{\mathrm{Al}}_2{\mathrm{O}}_3=2\times 27+3\times 16=102$

$0.051 \mathrm{g}\text{ of }{\mathrm{Al}}_2{\mathrm{O}}_3=\frac{0.051}{102}=0.0005 \mathrm{mol}$

$\text{ mole of }{\mathrm{Al}}_2{\mathrm{O}}_3\text{ contains }2\times 6.023\times {10}^{23}{\mathrm{Al}}^{3+}\text{ ions }$

$0.0005\text{ mole of }{\mathrm{Al}}_2{\mathrm{O}}_3\text{ contains }2\times 0.0005\times 6.023\times {10}^{23}$

${\mathrm{Al}}^{3+}\text{ ions }=6.023\times {10}^{20}{\mathrm{Al}}^{3+}\text{ ions. }$