Write the equation of the circle whose center is at the origin and whose diameter is 12.

The standard form of the equation of a circle is,
                        (x - h)² + (y - k)² = r²
where the center is at (h, k) and its radius is r. From the givens above, the equation of the circle is,
                         x² + y² = (12/2)²
or equal to x² + y² = 36. 

9/12 is the answer because 3/4 times 3 each is 9/12

Understanding what? lol