Determine the mass of an α -particle, given that its binding energy...

The $\alpha$-particle is a helium nucleus, which consists of $2$ protons and $2$ neutrons.

Mass of a proton, $m_p \approx 1.00727 \text{ u}$
Mass of a neutron, $m_n \approx 1.00866 \text{ u}$

The total mass of the constituent nucleons is:
$M_{\text{nucleons}} = 2m_p + 2m_n$
$M_{\text{nucleons}} = 2(1.00727 \text{ u}) + 2(1.00866 \text{ u})$
$M_{\text{nucleons}} = 2.01454 \text{ u} + 2.01732 \text{ u} = 4.03186 \text{ u}$

The mass defect $\Delta m$ is related to the binding energy (BE) by the relation:
$\Delta m = \dfrac{\text{BE}}{931.5 \text{ MeV/u}}$
$\Delta m = \dfrac{28.2 \text{ MeV}}{931.5 \text{ MeV/u}} \approx 0.03027 \text{ u}$

The mass of the $\alpha$-particle is the total mass of the nucleons minus the mass defect:
$M_{\alpha} = M_{\text{nucleons}} - \Delta m$
$M_{\alpha} = 4.03186 \text{ u} - 0.03027 \text{ u} = 4.00159 \text{ u} \approx 4.0016 \text{ u}$