You must set up two ratios, one for each photograph, and put them in the same order. The ratio of the unknown width, n, to the length of the first photograph is $\displaystyle\frac{{n}}{{24}}\usepackage{cmbright}\Huge\$ . The ratio for the width to length of the other photograph is $\displaystyle\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ . Set up a proportion that states that the two ratios are equal, and solve for n.