If L=length and B=breadth LB=210. Its perimeter is 2(L+B). Area is assumed to be 210 sq m.
L=210/B. So there are infinitely many solutions, but if L and B are integer metres, we need to break 210 into factors:
(1,210), (2,105), (3,70), (5,42), (6,35), (7,30), (10,21), (14,15)
The perimeters corresponding to these pairs of length and breadth are:
422, 214, 146, 94, 82, 74, 62, 58.