The equation $\displaystyle{{b}}^{{y}}={x}\usepackage{cmbright}\Huge\$ is equivalent to $\displaystyle{\log}_{{{b}}}{x}={y}\usepackage{cmbright}\Huge\$ , so: $\displaystyle{b}={9}\usepackage{cmbright}\Huge\$ , $\displaystyle{y}=-{3}\usepackage{cmbright}\Huge\$ , and $\displaystyle{x}=\frac{{1}}{{729}}\usepackage{cmbright}\Huge\$

Substitute these values into the logarithmic equation and you have: $\displaystyle{\log}_{{{9}}}{\left(\frac{{1}}{{729}}\right)}=-{3}\usepackage{cmbright}\Huge\$