23948

Firstly, you double the middle number/numbers (whether there is one middle number or more).
Secondly, you have to add all the numbers on the bottom layer, including the ones that are doubled.
This will equal the top number.

For example;

18

7 11
2 5 6

If you double the five, you get 10. Then you add 10 and 2 and 6 to get 18. THis is the top number.

This is because to get 7, you add 2 and 5.
To get 11, in this instant, you add 5 and 6.

So if we represent the 2 as y, the 5 as x, and the 6 as z.
7 = y + x
11 = x + z

Which means:

18 = y + 2x + z

So you double the middle number, and add the other numbers.
Any nubers in the middle, you double.