The formula for any geometric sequence is an = a1 · rn - 1 , where a n represents the value of the n th term, a1 represents the value of the first term, r represents the common ratio, and n represents the term number. what is the formula for the sequence -3, -6, -12, -24,

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