Correct.

$\displaystyle{y}={\log}_{{{3}}}{243}\usepackage{cmbright}\Huge\$ is equivalent to $\displaystyle{{3}}^{{y}}={243}\usepackage{cmbright}\Huge\$ .

Try different powers of 3: $\displaystyle{{3}}^{{2}}={9}\usepackage{cmbright}\Huge\$ , $\displaystyle{{3}}^{{3}}={27}\usepackage{cmbright}\Huge\$ , $\displaystyle{{3}}^{{4}}={81}\usepackage{cmbright}\Huge\$ , $\displaystyle{{3}}^{{5}}={243}\usepackage{cmbright}\Huge\$

So $\displaystyle{y}={5}\usepackage{cmbright}\Huge\$ .