Well, there is nothing much to explain about why it isn't exactly 3.14... [tex]\pi[/tex] is the ratio between the circumference and the diameter of a circle. If you perform this division, the result will not be exactly 3.14, never. It's just a fact.
As for part (b), the we just stated that
[tex]\pi=\dfrac{C}{2r}[/tex]
So, if we want to solve for [tex]C[/tex], we have
[tex]C=2\pi r[/tex]
If you want to use 3.14 for [tex]\pi[/tex] and the radius is 4, the circumference will be
[tex]C=2\cdot 3.14\cdot 4=25.12[/tex]
