Option Explicit

Private Declare Function ShowWindow Lib "user32.dll" (ByVal hwnd As Long, ByVal nCmdShow As Long) As Long
Private Declare Function FindWindow Lib "user32.dll" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long

Private Const SW_HIDE As Long = 0
Private Const SW_NORMAL As Long = 1

Dim lRet As Long

'Ocultar calculadora
Private Sub Command1_Click()
lRet = FindWindow(vbNullString, "Calculadora")

If lRet <> 0 Then
    Call ShowWindow(lRet, SW_HIDE)
End If
End Sub

'Mostrar calculadora
Private Sub Command2_Click()
    Call ShowWindow(lRet, SW_NORMAL)
End Sub