How do you find the area of diameter of a circle?

You misunderstand. 'The area of the diameter'. The diameter is a STRAIGHT LINE, it is NOT an area.

The question should read ' How do you find the area of a circle, knowing only the diameter?'

To answer the question

Remember the equation ;-

A = pi*r^2

Where

'A' is the area

'pi' ~ 3.14 (A constant)

'r' is the radius.

Now the radius is one half the diameter ; (r = d/2) or (d = 2r)

We can algebraically substitute

A = pi *(d/2)^2

So as an example , is the diameter is 20

Then

A = 3.14 ( 20/2)^2 (Remember ; do what's inside the brackets first)

A = 3.14 * 10^2

A = 3.14 * 100

A = 314.00