To calculate the mass of magnesium we must subtract the weight of the beaker and the magnesium minus the weight of the beaker, we do it as follows:
[tex]\begin{gathered} \text{Weight of Magnesium =}(\text{Weight of the beaker + Magnesium)}-(\text{Weight of the beaker )} \\ \text{Weight of Magnesium }=2.85g-1.86g \\ \text{Weight of Magnesium }=0.99g \end{gathered}[/tex]So, the weight of Magnesium is 0.99g