It appears that you added $\displaystyle{4}{y}\usepackage{cmbright}\Huge\$ and $\displaystyle{9}{y}\usepackage{cmbright}\Huge\$ as if they were like terms. $\displaystyle{4}{y}\usepackage{cmbright}\Huge\$ is a factor. You need to apply the distributive property to $\displaystyle{4}{y}{\left({9}{y}-{7}{x}{y}\right)}\usepackage{cmbright}\Huge\$ .