Correct. The first pair of terms has a GCF $\displaystyle{3}{a}\usepackage{cmbright}\Huge\$ and the second pair has a GCF 4:

$\displaystyle{3}{a}{b}-{15}{a}+{4}{b}-{20}={3}{a}{\left({b}-{5}\right)}+{4}{\left({b}-{5}\right)}={\left({b}-{5}\right)}{\left({3}{a}+{4}\right)}\usepackage{cmbright}\Huge\$

So $\displaystyle{3}{a}\usepackage{cmbright}\Huge\$ is the missing term.