Answer:
,

Explanation:
Given
rectangle with its base on x-axis
and other two corners at parabola
and parabola is downward facing symmetric about y-axis
let y be the y co-ordinate of the corner thus x co-ordinate is given by

Thus lengths of rectangle is
& y
Area

differentiating w.r.t to y for maximum area

we get y=

and

A_{max}=16.21 units