Question

Name the lengths of the sides of three rectangles that have a perimeter of 14 units

Answer 1

If you don't mind being around fractions and decimals, then there are an
infinite number of possible rectangles with the perimeter of 14.

If you'd rather just stick with whole numbers, then there are three choices:
 
--  1 unit by 6 units

--  2 units by 5 units

--  3 units by 4 units

Answer 2

Let W be the width and L be the length:

2W + 2L = 14

W + L = 7

(W,L) = (1,6), (2,5), (3,4), (4,3), (5,2), (6,1)