the
geometric sequence is defined as a(n+1)/an=q, q is not equal 0
so we can find easily q by a3/a2=a2/a1=12/-6= -6/3= -2
so the expresion is an=apq^n-p
where ap is the first term, for our case, ap=a1=3,so an =3(-2)^n-1
so the true answer is
an = 3(−2)^n − 1 where n ≥ 1