This expression is a perfect square trinomial because $\displaystyle{49}{{a}}^{{2}}={{\left({7}{a}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ , $\displaystyle{16}={{\left(-{4}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ , and $\displaystyle-{56}{a}={2}\cdot{7}{a}\cdot{\left(-{4}\right)}\usepackage{cmbright}\Huge\$ . Therefore, $\displaystyle{49}{{a}}^{{2}}-{56}{a}+{16}={{\left({7}{a}-{4}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ .