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\$ . The constant term must be a perfect square. Though 81 is a perfect square, $\displaystyle-{81}\usepackage{cmbright}\Huge\$ is not, so this is not a perfect square trinomial.