The first figure consists of one tile with perimeter $3 \times 7 \mathrm{~cm} = 21 \mathrm{~cm}$. Each time an additional tile is added, the perimeter of the figure increases by 7 cm (one side length of a tile), because one side length of the previous figure is 'covered up' and two new side lengths of a tile are added to the perimeter for a net increase of one side length (or 7 cm). Since the first figure has perimeter 21 cm and we are looking for the figure with perimeter 91 cm, then the perimeter must increase by $91 \mathrm{~cm} - 21 \mathrm{~cm} = 70 \mathrm{~cm}$. Since the perimeter increases by 7 cm when each tile is added, then $\frac{70 \mathrm{~cm}}{7 \mathrm{~cm} / \mathrm{tile}} = 10$ tiles need to be added to reach a perimeter of 91 cm. In total, this figure will have $1 + 10 = 11$ tiles.