The ratio of the width of the postcard to the unknown length, n, is $\displaystyle\frac{{4}}{{n}}\usepackage{cmbright}\Huge\$ . The ratio for the poster is $\displaystyle\frac{{24}}{{36}}\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.