Here's the question you clicked on:
Cherishyou
Write the equation of the circle whose center is at the origin and whose diameter is 12.
x^2 + y^2 = r^2 is the general formula for a circle with the center in [0,0] Thus, \[x^2 + y^2 = 12^2\] \[x^2 + y^2 = 144\]
the general form a a circle centered at \((a,b)\)\[(x-a)^2+(y-b)^2=r^2\]
This was (a little bit) of a trick question. They tell you the diameter is 12. But you need the radius, which is 1/2 of the diameter. The radius is 6