Although an answer has been accepted already, let me give another simple approach. You don't need to define a function just to evaluate the right hand side of your equation for different n-values. Mathematica can replace the n with numeric values simply be using the /.
operator. Say you want to know the t-values for n=3 and n=7, you can write
-800 n - 12000 /. n -> {3, 7}
(*
Out[4]= {-14400, -17600}
*)
These rules and replacements are a big thing in Mathematica and are worth a deeper look, because they work on any kind of expression. You should check Replace
, ReplaceAll
, ReplaceRepeated
, ReplaceList
and Rule
(and RuleDelayed
).