The GCF of the first two terms is x and the GCF of the last two terms is 7. Factor those out. The resulting terms have a common factor $\displaystyle{\left({3}{x}+{4}\right)}\usepackage{cmbright}\Huge\$ .

$\displaystyle{3}{{x}}^{{2}}+{4}{x}+{21}{x}+{28}={x}{\left({3}{x}+{4}\right)}+{7}{\left({3}{x}+{4}\right)}={\left({3}{x}+{4}\right)}{\left({x}+{7}\right)}\usepackage{cmbright}\Huge\$