Though 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\$ , when attempting to combine the numerators you probably forgot to distribute the minus sign in the second term:

$\displaystyle-\frac{{{\left({7}{{x}}^{{2}}-{49}{x}\right)}}}{{{\left({x}+{7}\right)}{\left({x}-{7}\right)}}}=+\frac{{-{7}{{x}}^{{2}}+{49}{x}}}{{{\left({x}+{7}\right)}{\left({x}-{7}\right)}}}\usepackage{cmbright}\Huge\$

After all numerators are simplified, you may combine them while keeping the denominator the same.