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use a proportion to find the missing side length, xx .
enter your answer in the box.
two rectangles. larger rectangle is located on the left with a top side length of 10 and a right side length of 6. smaller rectangle is located on the right with a top side length of 5 and a right side length of x.

If I understand correctly, the question is saying that the rectangles have the same proportions and wants to know the right side length of the smaller rectangle.
If this is the case, then the proportion is 5:3, with 3 being the right side length of the smaller rectangle.
This is because The original is 10:6, which is equal to 5:3.
Also, 5 is half of 10, so to find the right side length, just half the original as well, since you have to do the same to both sides. This results in lengths of 5 and 3.

Hope this helps :)

Top/side = top/side
10/6 = 5/x
Cross multiply
10x = 30
Divide both sides by 10
x = 3