Acircle has an area of 81 3.14 units . what is the diameter in terms of 3.14a circle has an area of 81 pi square units what is the diameter of the circle

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.