Answer
$a_n=(-1)(-3)^n.$
Work Step by Step
We know that $a_n=a_1r^{n-1}.$
where $r$=common ration (we can get this by dividing any two consecutive terms) and $a_1$= the first term
Here $a_6=a_1r^{6-1}=243\\a_1\cdot(-3)^5=243\\a_1=-1$
Hence $a_n=(-1)(-3)^n.$
