How do I solve sequences and how can I get the nth number?

e.g : 2,4,6,8..

the formula for the sequence can be found by using a+(n-1)d , where a is the first number in the sequence and d is the diffenece between the adjacent numbers in the sequence. so a =2 and d=2 so the formula is 2+(n-1)x2 = 2+2n-2 = 2n