The ratio of the unknown length, n, to the width of the photograph is $\displaystyle\frac{{n}}{{3}}\usepackage{cmbright}\Huge\$ . The ratio for the other photograph must be set up in the same order: length to width, or $\displaystyle\frac{{12}}{{9}}\usepackage{cmbright}\Huge\$ . Set up a proportion that states that the two ratios are equal, and solve for n.