the secret formula seems to be: 7 plus 2 plus 2 plus 2...
so 7+2....
but the plus 2 is repeated!
so
7+(2*x)
how many times is it repeated? 7 is the first term, 9 is the second, but it's repeated only once.. so the number of the term -1!
like this:
7+(2*(n-1))