Find the 1st and 2nd differences by subtracting each number
from the one below it and writing the difference by that
number:

 3   6   9 
 9  15   
24  

Put another 9 under the 9 in the 2nd differences
and work your way back to the first column by adding:

 3   6   9 
 9  15   9   
24  24
48

Answer: 48

Edwin