What area of the cheesecake is covered in chocolate icing?

Question

The top of a square cheesecake is entirely covered by a square of chocolate icing surrounded by piped cream.
If the sides of the top of the cake are 4 cm longer than the sides of the chocolate square, and the total area of the top of the cheesecake covered in cream is only 216 sq cm.
What area of the cheesecake is covered in chocolate icing?

Answer 1

625 sq sm

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(x+4)^2-x^2=216 => x=25 sm
x^2=625 sq sm