Whenever the value of 'n' is given in the form of an algebraic expression the common difference of the Arithmetic Progrsiion (A.P) is the coefficient of 'n' in the given expression.
Ex: In this Problem,
Given that n=6n+3
The coeeficient of 'n' is '6'
If you obsereve the answer you will also see that the Common difference is also '6'