an = a1 * r^(n - 1)....where a1 is the first term and r is the common ratio
to find the common ratio, take ur 2nd term and divide it by ur first term
so ur common ratio is : 8 / -2 = -4....this is basically saying that to find ur next term, u multiply by -4.
so ur rule is : an = -2 * -4^(n - 1) <==