# Help with arithmetic sequence

#### milkshakesxoxo

In an arithmetic sequence, the seventh term is 3 and the sixteenth term is 9. use arithmetic means to determine the common difference and the first term of the sequence

Thanks for the help!

#### romsek

MHF Helper
$a_k = a_1+(k-1) d$

$a_7 = a_1 + 6d = 3$

$a_{16}=a_1 + 15d = 9$

not sure about using arithmetic means but subtracting one equation from the other gets you

$9d = 6$

$d = \dfrac 2 3$

$a_1 = -1$