Maths

You can multiply the diamter by 3.14

Well to find circumference on a circle you must use the following formula:

C = 2 * 3.14 * r

So you understand the formula here is what each one means

r = radius

3.14 = pi 

Now you can solve:

Diameter = r*2

so you have a part of the equation

13 = 2 * r

So all you have to do is multiply by pi or 3.14

13 * 3.14 = 40.82

If you did it by radius the equation would be:

2 * 3.14 * 6.5 = 40.82

But would still have the same answer

Hope this helps ;P