hello!
so area of a square is calculated by this equation
s^2 = a
which means if you multiply two sides together you get the area
what you want to do now is to find a side of the square
plug in the 900 square meters
s^2 = 900
square root both sides to isoate the variable
s = 30 meters
this means that one side is 30 meters
since it is a SQUARE all four sides are equal
just multiply 30 by 4 and that will get the amount of fencing you'll need
30*4 = 1200 meters
you will need 1200 meters of fencing
WAIT
there is a possibility that one side of the house has already enclosed one side of the fence
so if that's the deal then
30*3 = 900meters
IF ONE SIDE OF THE HOUSE IS ALREADY ENLCOSING ONCE SIDE THEN YOU WILL NEED 900 meters of fencing.