Area is nothing but the region bounded by the shape of an object.Area is the two dimensional quantity.Different shapes have different areas and it has units square units(square metres,square centimetres etc)

In the above given problem,

length of square shaped playground(s)=300m

radius of circular swimming pool(r)=10m

area of the rest of the playground= area of the total play ground-area of the circular swimming pool

so,

area of the total square playground=s*s (area of square=side square)

=300*300 m^2

=9*10^4 m^2

Area of circular swimming pool=pi*r*r (area of circle=pi*r^2)

=(22/7)*10*10 m^2

=314.28 m^2

area of remaining playground=area of square playground-area of circular swimming pool

=90000m^2-314.28 m^2

=89,685.71 m^2

Hence area of remaining playground is 89,685.71 m^2