You know that the first rectangle has a width of 5 m and the similar triangle has a width of 15 m. The relationship between those widths can help you determine the length of the similar triangle. Since 5 x 3 is 15 the relation between these two rectangles would be x3 therefore if the length is 7 m then 7 x 3 is 21, the length of the similar triangle is 21.