If (big if) this is an arithmetic sequence, then the formula is y = 7n-1.
But there is no way to say for sure that this actually is an arithmetic sequence. There are millions of other formulas that will work for this sequence.
For just one example, the formula could be y = (-7n^4 + 70n^3 -245n^2 +434n - 180 ) / 12. That works perfectly.