a = ((4  (x^2))/3) - ((x^4)/3)

Plot[a, {x, -2, 2}]

liste = Table[{x, a}, {x, -2, 2}]

Lagrange[liste_, x_] := 
      Sum[liste[[k, 2]]*Product[x - liste[[j, 1]], {j, 1, k - 1}]*
          Product[x - liste[[j, 1]], {j, k + 1, 
                Length[liste]}]/(Product[
                  liste[[k, 1]] - liste[[j, 1]], {j, 1, k - 1}]*
                Product[
                  liste[[k, 1]] - liste[[j, 1]], {j, k + 1, 
                    Length[liste]}]), {k, 1, Length[liste]}]

a = Lagrange[liste, x]

Expand[a]

a = InterpolatingPolynomial[liste, x]

Expand[a]

liste = Join[{{0, -120}}, Table[{k, 0}, {k, 1, 5}]]

Lagrange[liste, x]