The definition is pretty simple:

So, we only have to sum the expressions:

Answer:
13
Step-by-step explanation:

Hope this helps..
Hey, there’s no circle for me to look at unfortunately!
Answer:
B (2,6,10,14)
Step-by-step explanation:
I think C and D are imposible to be the answer.
A and B are 50% to be the answer.
But I choose B.