Answer:
radius = 1.4
Explanation:
The formula for circumference is as follows:

or

However, these formulas are the same, because the diameter is equal to the radius times 2. I prefer to use the first (it gives you less values to work with).
Anyhow, We would substitute like this:

We know that pi, is going to be equal to: 3.14159
So we work from there...

Diameter divided by 2 would be the radius, and thats:
1.4006
or 1.4