I have to find the maximum area of a rectangle that already has a 100m fence in the ground, and I am given 200m to add onto it. However, my answer doesn't make sense:

A=b x h

P=300=2b + 2h

300=2(100+x)+2h

300=200+2x+2h

h=50-x

A=b(50-x)

A=50b-xb

dA/db = 50-x

0=50-x

x=50