Similar rectangles have or form similar ratios. Hence to solve this, you have to get the ratio of the measurements:
Let N be the length of the similar rectangle
5/7 = 15/N
5(N) = 7(15) -> cross multiply
5N = 105
5N/5 = 105/5 -> divide both sides of the equation by 5
N = 21
Therefore, 21 meters in the length of the similar rectangle
To check -
5/7 = 15/21 (cross multiply)
105 = 105