You add them and then divide by how many numbers there are.
a. (3+43)/2 = 23
b. 0
c. -1
1 step: 
,

, then

.
2 step: 
, then




and

will have form

.
3 step: Solve this system

and dividing first equation on second we obtain

. So,

and

,

- the common ratio.
4 step: Insert

into equation

and obtain

, from where

.
Ok so to answer this, we first need to figure out the area of both circles
we will use the formula A=πr²
so our two areas end up being 121π and 81π
now all we do is subtract the smaller from the larger
121π - 81π = 40π
shaded region = 40π
The answer is the forth one.