a) The answers you should have are perimeter=2(100+120)=440m. Area=12000sq m.
b) 100m rounded to the nearest metre could be between 99.5m to 100.5m and 120m between 119.5m and 120.5m. The largest perimeter would be 2(100.5+120.5)=442m (strictly < 442m) smallest is 2(99.5+119.5)=438m.
c) The largest area is 100.5*120.5=12110 sq m approx (<12110.25 sq m); the smallest=11890.25 sq m.