If the diameter of a circle is 14 cm what is the circumference of the circle?

You need to understand the concept of the number pi.

pi is the the number of diameters you can "wrap" around a circle.

The number is 3.14...

So if you had a circle with a diameter of 1 cm then the circumference would be 3.14 x 1 which equals 3.14 cm

Since the diameter is 14 cm the circumference is 3.14 x 14 cm

The equation that deals with this is ...

circumference = pi x diameter

or

c=pi x D