Starting with n=1 the general term for (a) is 7n+1. But if n starts at 0, the general term is 7n+8.
(b) The common difference is 5-4x. If n starts at 0 then the general term is 8+2x+(5-4x)n; but if n starts at 1 we have to step back a term: 3+6x+(5-4x)n.
(c) The common difference is -7, so we have 8-7n if n starts from 0 and 15-7n if starts at 1.