You must combine only like terms. If the problem asked for $\displaystyle{4}{y}+{3}{y}+{9}{y}+{6}{y}\usepackage{cmbright}\Huge\$ , that would simplify to $\displaystyle{22}{y}\usepackage{cmbright}\Huge\$ . In the given problem, $\displaystyle{4}{y}\usepackage{cmbright}\Huge\$ and $\displaystyle{9}{y}\usepackage{cmbright}\Huge\$ are like terms, while 3 and 6 form a separate group of like terms.