Step-by-step explanation:
every new term is created by adding a constant (I call here x) to the previous term.
a1 = b
a2 = c = a1 + x = b + x
a3 = a2 + x = a1 + 2x = b + 2x
so, we see
an = a1 + (n-1)x = b + (n-1)x
from a1 and a2 we know
c = b + x
x = c - b
so, ultimately we have
an = b + (n-1)(c-b)
