The ratio of the unknown length, n, to the width of the photograph is $\displaystyle\frac{{n}}{{3}}\usepackage{cmbright}\Huge\$ . The ratio of the length to the width for the other photograph is $\displaystyle\frac{{12}}{{9}}\usepackage{cmbright}\Huge\$ . Set up a proportion that states that the two ratios are equal, and cross multiply. Don't forget to divide by the number in front of n.