Correct. For the polynomials to be the square of a sum or the square of a difference, the first and last terms both have to be perfect squares. You correctly identified the polynomials that were in the form $\displaystyle{{a}}^{{2}}+{2}{a}{b}+{{b}}^{{2}}\usepackage{cmbright}\Huge\$ and $\displaystyle{{a}}^{{2}}-{2}{a}{b}+{{b}}^{{2}}\usepackage{cmbright}\Huge\$ in this group.