Arectangle has a width of 5 meters and a length of 7 meters a similar rectangle has a width of 15 meters what is the length of the similar rectangle?

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