Correct. The terms $\displaystyle{10}{a}{b}\usepackage{cmbright}\Huge\$ , $\displaystyle-{5}{b}{a}\usepackage{cmbright}\Huge\$ , and $\displaystyle{4}{a}{b}\usepackage{cmbright}\Huge\$ are like terms. They can be combined this way: $\displaystyle{10}{a}{b}-{5}{b}{a}+{4}{a}{b}={10}{a}{b}-{5}{a}{b}+{4}{a}{b}={\left({10}-{5}+{4}\right)}{a}{b}={9}{a}{b}\usepackage{cmbright}\Huge\$