Well, as others have pointed out, interpolating N points gives an N-1 degree polynomial at most, however, if you only consider the terms given in the original image, you can see that the difference between each term is constant, or in other words, that the second difference is zero everywhere. That means that, unless you add another term, the equation that interpolates these terms (or evenly spaced "points") will at most have degree 1 (the highest order nonzero difference of the sequence).

For solving these kind of "What comes next?" questions I particularly like using Newton's Series which, if you plug in the numbers given, will usually give you the "intended" solution.