Let the breadth of the rectangle = b mts
Then Length of the rectangle = b + 6 mts
Given perimeter = 84 mts
2(L + B) = 84 mts
2(b+6 + b) = 84
2(2b + 6) = 84
4b + 12 = 84
4b = 84 - 12
4b = 72
b = 18 mts
=> Length = b + 6 = 18 + 6 = 24 mts
Now, required Area of the rectangle = L x B = 24 x 18 = 432 sq. mts