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The formula to calculate circumference is πd or 2πr.  We also know that 2r = d.

So, using these formulas, let's try to figure out the first row of the table.

The diameter is 14.

We know that 2r = d or 2r = 14, therefore r=7.  This means the radius is 7.

The circumference is πd or 14π or ≈ 43.96.

Let's move to the second row.

We know that the radius is 21.

2r = d
2(21) = d
d = 42
The diameter is 42.

Therefore, the circumference is 42π or ≈ 131.88

Let's move to the final row.

We know that the radius is 10.5.

2r = d
2(10.5) = d
d=21

Therefore, the diameter is 21 cm.

Circumference = πd = 21π or ≈ 65.94