The ratio of the unknown width, n, to the length of the photograph is $\displaystyle\frac{{n}}{{24}}\usepackage{cmbright}\Huge\$ . The ratio for the other photograph is $\displaystyle\frac{{3}}{{4}}\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.