I will give brainliest if you get this right.

5 cm

Explanation:

First, let's find the perimeter of the rectangle.

The perimeter of a rectangle is:

p=2l+2w

In this rectangle, we know that the length is 7 centimetres and the width is 3 centimetres.Therefore, we can substitute 7 in for l and 3 for w.

p=2(7)+2(3)

Multiply 2 and 7

p=14+2(3)

Multiply 2 and 3

p=14+6

Add 14 and 6

p=20

We know the perimeter of the rectangle is 20 centimeters. We also know that the perimeter of the square is equal to the perimeter of the rectangle.

The perimeter of a square is:

p=s+s+s+s or p=4s

We know that the perimeter of the square is equal to the rectangle. The rectangle's perimeter is 20. Therefore, we can substitute 20 in for p.

20=4s

Now, we want to find what the side length is. We have to get s by itself. 4 and s are being multiplied. To reverse this, divide both sides by 4.

20/4=4s/4

20/4=s

5=s

The side length of the square is 5 centimeters.