An infinitely long cylinder of radius a in free space is charged with a volume charge density p(r)=p0(a-r)/a (0<=r<=a), where p0 is a constant and r the radial distance from the cylinder axis. Find the Charge per unit length of the cylinder.

In the problem a volume charge density was given. The only definition I can find is:

p=dQ/dv. Knowing this I can integrate the volume charge density given to get the charge, however, I'm unsure how to come to charge per unit length. Any help appreciated.