You must combine only like terms. If the problem asked for $\displaystyle{3}+{2}+{7}+{5}\usepackage{cmbright}\Huge\$ , that would simplify to 17, as ordinary numbers are all like terms. In the given problem, $\displaystyle{3}{y}\usepackage{cmbright}\Huge\$ and $\displaystyle{7}{y}\usepackage{cmbright}\Huge\$ are like terms, while 2 and 5 form a separate group of like terms.