There is no simple answer to this. You must get used to the notation used of which there are slight variances of the letters used depending on preference.
Counting using the letter i to designate position in the sequence and the letter n for the last term position. Another time the letter n could be use instead of i.. So you could have the nth term, ith term or even perhaps using the letter t
Of sequence and series the latter is the hardest (adding up a sequence). For this the only real solution is to find a guide/book with lots of examples and practice questions (with answers).
Sequences follow two basic rules. One for Arithmetic and another for Geometric.
Let the first value be
Let the difference between terms be
Let the ith term be
Notice the pattern!
So for any
You can determine
You sometimes have alternating positive and negative in which case you would have some variant on:
Normally you find people use the letter
You can determine r by division.
These you just have to practice as there are many varients.