How to find radius from circumference

The circumference of a circle is equal to pi times the diameter. The diameter is two times the radius, so the equation for the circumference of a circle using the radius is two times pi times the radius.

Answer is A. y=-6 is a horizontal line that runs through (0,-6). It has no slope because slope=rise/run ( As they teach in school, but the slope can be thought of as the hypotenuse of a triangle ).

Since there is no “rise” but is only “run”, such that slope=0/anything is 0. ¿Comprendes?