A perfect square trinomial is in the form $\displaystyle{{r}}^{{2}}+{2}{r}{s}+{{s}}^{{2}}\usepackage{cmbright}\Huge\$ or $\displaystyle{{r}}^{{2}}-{2}{r}{s}+{{s}}^{{2}}\usepackage{cmbright}\Huge\$ . Although $\displaystyle{16}{{x}}^{{2}}={{\left({4}{x}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ and $\displaystyle{9}={{3}}^{{2}}\usepackage{cmbright}\Huge\$ , the middle term, $\displaystyle{12}{x}\usepackage{cmbright}\Huge\$ , is not equal to $\displaystyle{2}\cdot{4}{x}\cdot{3}\usepackage{cmbright}\Huge\$ , so this is not a perfect square trinomial.