You may have found the value of $\displaystyle{\log}_{{{10}}}{10},{000}\usepackage{cmbright}\Huge\$ to be 4, which is correct, but you did not use the product property, $\displaystyle{\log}_{{b}}{\left({M}{N}\right)}={\log}_{{b}}{M}+{\log}_{{b}}{N}\usepackage{cmbright}\Huge\$ . Use the product property and then simplify.