You probably correctly rewrote the fractions using the LCD, but made a mistake simplifying the expression on the last step. When subtracting $\displaystyle\frac{{{4}{y}+{24}}}{{{\left({y}-{6}\right)}{\left({y}+{6}\right)}}}\usepackage{cmbright}\Huge\$ , be sure to distribute the minus sign to $\displaystyle{4}{y}\usepackage{cmbright}\Huge\$ and 24. You added the 24.