Find the first four terms of each of the recursively defined sequences

How many 9 digit sequences have the digits 1,3 and 7 appearing at least once?

Please teach me how to show this step by step!

Use iteration to find a closed formula for the sequences below. Simplify your answer whenever possible.  a.) Let (a_n)be the sequence defined by the recursive relation an=3a_(n−1)+4, with initial term a0=3. b.)  Let (a_n) be the sequence defined by the recursive relation an=a_(n−1)+6, with initial term a0=5 c.) Let (a_n)be the sequence defined by the recursive relation an=a_(n−1)+5n+1 with initial term a0=0. d.)  Let (a_n)be the sequence defined by the recursive relation an=(a_(n−1)) / (1+a_(n−1)) with initial term a0=1.