Alright you find this by first multiplying 18•7=126 to find the amount of space taken up by the cards themselves and then you subtract that from the total distance 279-126=153 to find the amount of total distance between all the cards and then you can see from the image they give that there’s a space after every card Except the 18th so 153/17=9 gives you the amount of distance between each card.
The answer is 9cm.
Hope this helps :)