What are the instructions for what you are to do with it?

a4=24, r=2

That means you have a sequence of numbers.

__, __, __, 24, __. __, ... 

a4 means the 4th number in the series is 24,
The r=2 tells you that to get the number just to the right of
24, you multiply 24 by 2, and get 48

__, __, __, 24, 48. __, ...


Then yopu multiply 48 by 2 and get 96 next,

__, __, __, 24, 48. 96, ...

and so on.

To get the number just left of 24, you divide 24 by 2 and get 12.


__, __, 12, 24, 48, 96, ...

To get the number before that you divide 12 by 2 to get 6


__,  6, 12, 24, 48, 96, ...

Divide by 2 again and get 3 for the first term a1

 3,  6, 12, 24, 48, 96, ...

I don't know what you're supposed to do since you didn't give
the instructions given in your book.

There are formulas for finding specific terms, and adding up
specific numbers of these numbers.

This is called a "geometric sequence" (or progression).  You
always get the next term by multiplying a term by 2.

Edwin