You'd be better off if you'd invest a bit more time into presenting that formula for a geometric sequence. rn-1 is 'way off base.
Instead, try making the subscripts and superscripts stand out:
n-1
a = a * r
n n-1
Believe me, it's worth the extra work to do this!!
Given the sequence -3, -6, -12, -24, you can easily see that the first term,
a is -3. The common factor is 2. Note how (-3)*2=-6, and so on.
1
So, a = -3 and r=2.
1
So the formula for this geometric sequence is
n-1
a = -3*2
n
