BUT
it is in principle impossible to guess a missing number, because the missing number could have ANY value, depending on the function used to create the non-missing ones.
Look at this
data = Transpose[{Range[8], {1, 8, 27, 64, 125, y, 343, 512}}]
Now define
f[x_] := Plus @@ Table[
data[[i, 2]] Times @@ Table[
If[j != i, (x - data[[j, 1]])/(data[[i, 1]] - data[[j, 1]]), 1]
, {j, 1, 8}]
, {i, 1, 8}]
Then this f will reproduce all the numbers given
f/@Range[8]
and you may insert for the missing y ANY value you want!
And of course you can as alternative get the given numbers by { x, x^3 }.