To determine if Miriama is correct or not, you calculate the circumference of a circle.
Use the following formula for the circumerence C:
C = 2πr
where r is the radius of the crcumference. The radius is one half the diamater of a circle, and you have the diameter = 12cm. Then, the radius of the circle is
r = 12cm/2 = 6cm
Next, replace the value fo r into the formula for C:
C = 2π(6cm) ) 12π cm
Hence, Miriama is wrong and the circumference of the circle is 12π cm and not 6π cm.