(bn);...;1;x;9;-27; x-?

B1 = 1, b2=x, b3=9, b4=-27 q=b2/b1 = b3/b2=b4/b3=.........,q=-27/9 =-3 b2=b1.q,b2= 1.(-3)=-3 x=b2, x = -3           -------