Answer:

Step-by-step explanation:
A circle centered at (0,0) with radius r is 
Since your circle has diameter of 12, then its radius is 6. Then 
So a possible answer is: 
If you want to move the location of the center, but keep it on the x-axis, then add or subtract a number to the x, such as this:

The center of this circle would be (-4, 0) which is still on the x-axis.
Answer:
AREA:

CIRCUMFERENCE:

Step-by-step explanation:
The formula that is used to calculate the area of a circle is:

Where "r" is the radius of the circle.
The formula for calculate the circumference of a circle is:

Where "r" is the radius of the circle.
Then, you need to find the radius. This is:

Substituting the radius, you get that the area and the circumference are:
AREA:

CIRCUMFERENCE:

The answer might be slope.
Well, after doing all that math, here is what I came up with, I hope this helps! :) -1600x^2+28265x-21830