A trinomial in the form $\displaystyle{{r}}^{{2}}-{2}{r}{s}+{{s}}^{{2}}\usepackage{cmbright}\Huge\$ is a perfect square. Since $\displaystyle{49}{{x}}^{{2}}={{\left({7}{x}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ , $\displaystyle{25}={{5}}^{{2}}\usepackage{cmbright}\Huge\$ , and $\displaystyle-{70}{x}=-{2}\cdot{7}{x}\cdot{5}\usepackage{cmbright}\Huge\$ , this is a perfect square.