Here's a ways that always works:

Make a difference pyramid. Subtract every number that has 
a number just to the right of it from that number, putting 
the answer between them on the row below.  Continue until 
there is only one number along the bottom row.

5    17      22     26
  12      5      4
     -7      -1
          6


Repeat that number in this case 6, at the bottom and work 
backward by adding each number to the number on its upper
left until you get back up to the top row.  

5    17      22     26      35
  12      5      4       9
     -7      -1     5
          6      6

Answer: 35

Edwin