The Change of Base formula is: $\displaystyle{\log}_{{{b}}}{x}=\frac{{{\log}_{{{a}}}{x}}}{{{\log}_{{{a}}}{b}}}\usepackage{cmbright}\Huge\$

Using a new base of 10, you can rewrite $\displaystyle{\log}_{{{3}}}{42}\usepackage{cmbright}\Huge\$ as: $\displaystyle\frac{{{\log{{42}}}}}{{{\log{{3}}}}}\usepackage{cmbright}\Huge\$

You can then use a calculator to evaluate the quotient.