Find the 6th\displaystyle {6}^{{{t}{h}}} term of the geometric sequence:

 {243a(b),81a(2b),27a(4b),9a(8b),}\displaystyle {\left\lbrace{243}{a}{\left(-{b}\right)},{81}{a}{\left(-{2}{b}\right)},{27}{a}{\left(-{4}{b}\right)},{9}{a}{\left(-{8}{b}\right)},\ldots\right\rbrace}

 a6=\displaystyle {a}_{{6}}=