You have posted 2 separate questions here. Please, next time, separate them with a blank line:
A circle has an area of 81 3.14 units . what is the diameter in terms of 3.14?
I'd prefer you write this as
"A circle has an area of 81 pi units . what is the diameter in terms of pi?
Area of a circle = A = pi*r^2. Note that r= diameter / 2. Here, the area is 81 pi square units, or 9^2 * pi square units, which means that the radius is 9 units. The diameter is then 2(9 units) = 18 units.
Your second question, separated from the first, is:
"a circle has an area of 81 Pi square units what is the diameter of the circle?"
Use the following formula or formulas: d = 2*r; A = pi*r^2; A = pi*(d/2)^2. Find r first, and then find d.
