It's very simple. Ammonium carbonate is a solute and water is a solvent.
So, Mass percent of solute = [tex]\frac{Mass \hfill of \hfill solute}{Mass \hfill of \hfill solution} \times 100[/tex]
= [tex] \frac{33.2g}{33.2g + 39.5g} \times 100[/tex]
= 45.7%
