want to Multiply Some DataGridViewTextBox Values plz Suggest

Question

For ex:- I Have two DataGridViewColumns

 

As:- 1) Quantity_DataGridViewComboBoxColumn

        2)Price_DataGridViewComboBoxColumn

 

And Another Column Total_TextBoxColumn that I set it ReadOnly For Showing Result

 

I just Want to Multiply The Above Two Columns And Display result in Third One Total_TextBoxColumn

 

 

 

plz Suggest me Any As aEarly as Possible... :-)