Hello there!
The formula for finding the circumference of a circle is 2[tex] \pi [/tex]r (or 2*3*r in your terms). Although we don't have the radius yet, we can certainly find it pretty easily. Keep in mind that the radius will always be half the diameter. So, let's find the radius.
8 / 2 = 4; now we have everything we need to solve for the circumference.
2*3*4 is our expression. Now let's simplify it.
2 * 3 = 6
6 * 4 = 24
Your circumference is 24cm.
I hope this helps!
