You may have done the correct first step: $\displaystyle{F}-{32}=\frac{{9}}{{5}}{C}\usepackage{cmbright}\Huge\$

However, to isolate the variable C, you must now multiply both sides by $\displaystyle\frac{{5}}{{9}}\usepackage{cmbright}\Huge\$ , not $\displaystyle\frac{{9}}{{5}}\usepackage{cmbright}\Huge\$ .