[ADAM12]

و عليكم السلام و رحمة الله
استبدل الاسطر لديك بهذه 

كود :

Dim First_Total As Decimal = 0.0
       Dim Second_Total As Decimal = 0.0
       Dim Third_Total As Decimal = 0.0

       If Not TextBox1.Text = "" And Not TextBox2.Text = "" And Not TextBox3.Text = "" Then
           First_Total = TextBox1.Text
           Second_Total = TextBox2.Text
           Third_Total = TextBox3.Text
       End If
       
     
       Me.TextBox4.Text = String.Format("{0:N2}", First_Total - Second_Total + Third_Total)