The first two terms have a GCF $\displaystyle{5}{x}\usepackage{cmbright}\Huge\$ and the second two terms have a GCF $\displaystyle-{6}\usepackage{cmbright}\Huge\$ . Factor those out. The resulting terms have a common factor $\displaystyle{\left({x}+{1}\right)}\usepackage{cmbright}\Huge\$ .

$\displaystyle{5}{{x}}^{{2}}+{5}{x}-{6}{x}-{6}={5}{x}{\left({x}+{1}\right)}-{6}{\left({x}+{1}\right)}={\left({x}+{1}\right)}{\left({5}{x}-{6}\right)}\usepackage{cmbright}\Huge\$