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Find the formula for the arithmetic sequence with the first term $a_{1}=-3.1$ and the sixth term $a_{6}=6.4$ .

$a_{n}=-3.1+6.4(n-1)$

$a_{n}=-3.1+1.9(n-1)$

$a_{n}=6.4 + 1.9(n-1)$

$a_{n}=-3.1+1.9n$

none of the above