It is a unit of measurement. It seems like an incomplete question. 3.9cm is 3.9cm. If your asked to convert it to something then it looks like this.
1inch=2.54cm
x inches = 3.9cm
[tex]\frac{3.9}{2.54}= 1.5[/tex] inches
Hope that helps. For a better answer I would need what the question is asking.
