I am assuming that you know how to insert code in the code window and use it. Create three text boxes on your slide by going to Insert>Text Box. Do not use the Text Box in the Active X Control tool box (this is located on the Developer Tab of the ribbon).Then enter this code in a new module or the code window for the slide:

Sub test()

ActivePresentation.Slides(1).Shapes(3).TextFrame.T extRange.Text = _
CInt(ActivePresentation.Slides(1).Shapes(1).TextFr ame.TextRange.Text) _
+ CInt(ActivePresentation.Slides(1).Shapes(2).TextFr ame.TextRange.Text)


End Sub


This assumes that you are running the code on Slide 1 of your slideshow. If not, change the slides(1) to whichever slide number you want the code to run on.

If you don't know how to insert code in the VB Editor then look at this link http://www.gmayor.com/installing_macro.htm