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{9}{{x}}^{{2}}={{\left({3}{x}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ and $\displaystyle{4}={{2}}^{{2}}\usepackage{cmbright}\Huge\$ , the middle term, $\displaystyle{6}{x}\usepackage{cmbright}\Huge\$ , is not equal to $\displaystyle{2}\cdot{3}{x}\cdot{2}\usepackage{cmbright}\Huge\$ . So this is not a perfect square trinomial.