You may have correctly rewritten the terms as equivalent fractions with least common denominator $\displaystyle{\left({x}+{7}\right)}{\left({x}-{7}\right)}\usepackage{cmbright}\Huge\$ and combined the numerators while keeping the denominator the same. However, you cannot cancel a term of 49 on the top with a $\displaystyle-{49}\usepackage{cmbright}\Huge\$ on the bottom. You need common factors to simplify.