Let the length of the field be 4x m
Then, its breadth = 3x
∴area of the rectangular park = ((4x) × (3x)) = 12x2 m2
But, area = 1728m2 (from the question)
∴12x2 = 1728
= x2 = 1728/12
= x2 = 144
= x = √144
= x = 12
∴length = (4 × 12) = 48m, breadth = 3 × 12 = 36m
∴perimeter of the field = 2(l + b) units
= 2 (48 + 36)
= 2 (84)
= 168 m
∴the cost of fencing = ₹ (168 × 30)
= ₹ 5040