You may have reversed the x and y values when substituting. If you substitute in 1 for x and 12 for y, you will get $\displaystyle{12}={\log}_{{{12}}}{1}\usepackage{cmbright}\Huge\$ . This is equivalent to $\displaystyle{{12}}^{{12}}={1}\usepackage{cmbright}\Huge\$ , which is a false statement.