Вычислить значение выражения

Private Sub Command1_Click()
   Dim x As Single, xn As Single, dx As Single, xk As Single
   Dim a As Single, z As Single, k As Integer, s As String
   Dim P As Double, Sum As Double, F As Double
   xn = Val(Replace(InputBox("Введите начало отрезка (xn)", "Ввод данных", 2), ",", "."))
   xk = Val(Replace(InputBox("Введите конец отрезка(xk)", "Ввод данных", 6), ",", "."))
   dx = Val(Replace(InputBox("Введите шаг (dx)", "Ввод данных", 0.2), ",", "."))
   a = Val(Replace(InputBox("Введите значение коэффициента a", "Ввод данных", "0.9"), ",", "."))
   P = 1
   For x = xn To xk Step dx
       z = 5 * (x ^ 2) + (a ^ (1 / 3)) * Sqr(a ^ 5 + 3.8 * 10 ^ 6)
       P = P * z
       Sum = Sum + z
       s = s & "x = " & Round(x, 2) & vbTab & "z = " & Round(z, 2) & vbCrLf
   Next x
   F = P + Sum
   MsgBox s
   MsgBox "F = " & F
End Sub