Let:
Wi = Initial weight
Wf = Final weight
x = amount of mass the substance lost
Therefore:
[tex]\begin{gathered} Wi-x=Wf \\ where\colon \\ Wi=20.609 \\ Wf=17.39 \end{gathered}[/tex]solve for x:
[tex]\begin{gathered} x=Wi-Wf \\ x=20.609-17.39 \\ x=3.219g \end{gathered}[/tex]