This question was addressed to me by my school teacher while I was bunking the class.
Enquiry is from Corner Reflector in chapter Reflector Antenna of Antennas
For explanation I would say: In a corner reflector antenna, side length of reflector sheet and l=2d
Where, d is distance between feed and vertex of the reflector.
Given d=2m so l=2*2=4m=400cm.