Answer :
2x+y=400
A=xy area of farmland
y=400-2x
A=400x-2x^2
A'(x)= 400 -4x derivative of area with respect to x
0=400-4x find out where f ' equals 0
4x=400
x=100
A'(x)= 400 -4x
A''(x)= -4 check 2nd derivative. if negative then relative max at x=100
2x+y=400
y=200
dimensions 100x200
hope it helped