Question 1196659
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You have {{{ a*r^0 }}} = -3
and {{{ a*r^2 }}} = -108

so   {{{ r^2 = -108/-3 = 36 }}} -->  r = 6,  this is the common ratio (the number by which you multiply {{{a[n]}}} to get {{{a[n+1]}}} )

The missing term is {{{a*r^1}}} = {{{ -3*6 = -18 }}}

{ -3, -18, -108 }