Can someone show me the working for this please

A rectangular based area is fenced using 40m of fencing

Taking x metres for its length, show that the area of the rectangle,
A m^2, can be written

A = 100 - (x - 10)^2 . What is the maximum floor area, and the
corresponding length and breadth.

I know the answer is A=100m^2 , length = 10m, breadth = 10m

This question is in relation to questions on quadratic function f(x) =
ax^2+bc+c i have been doing

TIA

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