Add TextBox Entry To ComboBox Range

I have a user form that has a combobox, two textboxes, and a button on it. When the form is loaded, the combobox fills with data from a worksheet I created. THe worksheet has a column with the item names, and another column with the quanity of each item. The combobox is filled with the item names. SO far I have this done. My issue is that when the user selects an item from the combobox, and enters a quantity in to the first textbox, and then clicks the button, I want the quantity in the textbox to add to the quantity cell that the part number from the combobox references to. Then I want this new quantity to show in the cell and the second textbox.