What I’d the equation of a line that is perpendicular to -x+3y=9
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If the series is made up of numbers in an arithmetic progression, you have
where
is the first term and 
is the common difference between terms.
Since the last term in the series is
, you have
Solve the system
and you'll find that the first term is
(with 
).
where
Since the last term in the series is
Solve the system
and you'll find that the first term is