1) Resistance 'R' of a conductor is directly proportional to its length 7' at constant temperature and cross sectional area.
R $\propto$ l (i)
2) Resistance 'r' of a conductor is inversely proportional to its cross section area at constant temperature and length.
R $\propto$ 1/A --------(ii)
From (1) and (2)
R $\propto$ 1/A
=====> R = p l/A
Where p is a proportionality constant and p is called specific resistance or resistivity.