Using Avogadro's law, we know the amount of moles in the sample is
[tex]\frac{6.87 \times 10^{23}}{6.022 \times 10^{23}}=1.1408170043175 \text{ mol}[/tex]
Therefore, the atomic mass is:
[tex]\frac{26.1}{1.1408170043175}=\boxed{22.9 \text{ g/mol (to 3 sf)}}[/tex]
