Recurrence Relation Help?

A sequence a1, a2, a3, is defined by
A1 = k, an+1 = 4an - 7 where k is a constant
A) write down expression for a2 in terms of k.
B) find a3 in terms of k, simplify.
C) given a3 = 13, find value of k.

Sorry this is so confusing any help would be great on how to work it out