You may have thought that you could combine unlike terms by adding exponents. When you multiply you add exponents. When you add or subtract, you can only combine terms with the same variable and exponent. The given polynomial equals $\displaystyle{\left({8}{{a}}^{{2}}-{4}{{a}}^{{2}}\right)}+{\left({7}{a}-{9}{a}\right)}+{7}{{a}}^{{0}}\usepackage{cmbright}\Huge\$ , and you need to combine the like terms in the parentheses.