What is is circumference of a circle is 6.28. What is the area of the circle?

Your English grammar appears to be very poor.

I think you mean ' If the circumference of a circle is '6.28' , then what is the area?'.

Remember the two circle equ'ns.

C = 2 pi r

A = pi r^(2)

Since we know 'C' at 6.28, and pi = 3.14

Then we can find 'r' (Radius).

Hence

6.28 = 2 x 3.14 x r

r = 6.28/( 2(3.14))

r = 1 (one)

Substituting into 'A'

A = 3.14 (1)^(2)

A = 3.14 units ^(2).

That's doing the calculation in two steps. However, by algebraic reARRANGEMENT WE CAN DO IT IN ONE STEP. However, be careful with the algebra.

C= 2 pi r

r = C/2pi

r^(2) = (C/2pi)^(2)

Substituting

A = pi (C/2pi)^(2)

Cancel down by 'pi'

Hence

A = [C/2]^(2) / pi

A = [ 6.28 / 2 ] ^(2) / 3.14

A = [39.4383/4] / 3.14

A = [9.8596]/3.14

A = 3.14 As before.