___   ___   26    40    ___   the 3rd and 4th terms are 26 and 40
      ___   ___   14    ___      The only thing we know about the first differences is that one of them is 14
          4     4     4          we know the second differences are a constant 4

You can use the constant second differences to work both forward and backward in the sequence to determine the first differences (the successive first differences have a constant difference of 4)...

   ___   ___   26    40    ___
        6    10   14    18
          4     4     4

And finally you can similarly use those first differences to determine the original sequence.

    10    16   26    40    58
        6    10   14    18
          4     4     4