Answer:
x = 8
Explanation:
The complete question illustrates that the rectangle has sides ABCD and I've represented it as an attachment.
We have that:
AB = 3x
AD = 7x
The perimeter is calculated as:
Perimeter = 2 * (AB + AD)
Substitute values for AB, AD and Perimeter
160 = 2 * (3x + 7x)
160 = 2 * 10x
160 = 20x
Divide both sides by 20
8 = x
x = 8