Identify a1 and r for the sum of the geometric series.

-4, 8, - 16, 32, ...

A₁ is very clear, just the first term, namely - 4.

now, "r" or the common ratio, for a geometric serie/sequence, you can always tease it out by simply getting the quotient of the "next term by the previous term", in this case, say 8 / (-4), or (-16) / 8 and so on.

recall that a geometric sequence is done by using the common ratio to get the next term, thus the quotient of both terms is, yes, you guessed it, is "r", the common ratio.