Perimeter is the sum of all 4 walls.
Divide the perimeter in half for the total of the width + the length:
26/2 = 13 meters
Subtract the difference in length:
13 - 3 = 10
Divide by 2 walls:
10/2 = 5
The width is 5 metes.
The length is the 5 metes + the 3 meter difference = 5 +3 = 8 meters.
The length is 8 meters and the width is 5 meters.