Capacitance of a coaxial cable with a dielectric

A coaxial cable has an inner conductor of radius $a=1.0\mathrm{mm}$ and an outer conductor of inner radius $b=3.5\mathrm{mm}$; the space between the conductors is filled with polyethylene, with ${\epsilon }_r=2.3$. Compute the capacitance per unit length and the capacitance of a $25\mathrm{m}$ length of cable.