You may have tried to multiply both sides by the common denominator $\displaystyle{\left({x}-{5}\right)}{\left({x}+{3}\right)}\usepackage{cmbright}\Huge\$ to clear the fractions, but ended up with the incorrect equation $\displaystyle{12}{\left({x}-{5}\right)}={4}{\left({x}+{3}\right)}\usepackage{cmbright}\Huge\$ . When you have a fraction such as $\displaystyle\frac{{{12}{\left({x}-{5}\right)}{\left({x}+{3}\right)}}}{{{x}-{5}}}\usepackage{cmbright}\Huge\$ , it is the common factor in the numerator and denominator that will cancel, because $\displaystyle\frac{{{x}-{5}}}{{{x}-{5}}}\usepackage{cmbright}\Huge\$ simplifies to 1.