Question 439206
{{{a[4] = -18}}}

{{{54 = a[5]}}}

==> {{{r = a[5]/a[4] = 54/(-18) = -3}}}.

==> {{{a[4] = -18 = a[1]*(-3)^3 = -27a[1]}}}
==. {{{a[1] = (-18)/(-27) = 2/3}}}