23924

Key: numbers stand as x

8x+12
4x+4 4x+8
2x+1 2x+3 2x+5
x x+1 x+2 x+3

if you add x with x+1 you get 2x+1
add x+1 with x+2 you get 2x+3 and add x+2 with x+3 you
get 2x+3.

you do the same with the second stage to get the answers of the third stage, and you do the same with third stage to get the top answer.

so if x stands for 6 the pyramid will be like this:

60
28 32
13 15 17
6 7 8 9