$\displaystyle{25}{{x}}^{{2}}={{\left({5}{x}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ and $\displaystyle{36}={{6}}^{{2}}\usepackage{cmbright}\Huge\$ , so this is a difference of squares. Use $\displaystyle{{r}}^{{2}}-{{s}}^{{2}}={\left({r}-{s}\right)}{\left({r}+{s}\right)}\usepackage{cmbright}\Huge\$ to get $\displaystyle{25}{{x}}^{{2}}-{36}={\left({5}{x}-{6}\right)}{\left({5}{x}+{6}\right)}\usepackage{cmbright}\Huge\$ .