The value of y (breadth of rectangular field) is :

Let length = x

let breadth = y

from question $(x-50)(y+50)=xy$

so $xy-50x+50y-2500=xy$

$x-y=50$   ....(1)

$(x-10)(y-20)=xy-5300$

$⇒xy-20x-10y+200=xy-5300$

$2x+y=550$   ...(2)

eq. (2) + eq. (1)

$⇒3x=600⇒x=200m$

so from (1) $y = 150m$