I am getting 2 errors when I subtract 2 textbox values

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C#

MessageBox

TextBoxBase

Hello Guys,

I am getting 2 errors when i subtract 2 textbox values and shows result in message box so i get this 2 errors

Error	1175	The best overloaded method match for 'System.Windows.Forms.MessageBox.Show(string)' has some invalid arguments	20208	25	NewPOS

Error	1176	Argument 1: cannot convert from 'double' to 'string'

What I have tried:

double a1, a2, a3;
double.TryParse(txtTotal.Text, out a1);
double.TryParse(textBox73.Text, out a2);
a3 = a1 - a2;
MessageBox.Show(a3);