Answer is Field B is 400 m larger in area than Field A
Field A perimeter is 2(70) + 2(30) = 200
Field B is a square with the same perimeter of 200. Since the four sides are equal, we can divide 200 by 4 sides = 50 m per side.
Now we can find area
A = L x W
Field A = 30 x 70 = 2100
Field B = 50 x 50 = 2500
Subtract to get the difference
2500 (B) - 2100 (A) = 400 m
Field B is 400 m larger in area than Field A