Embedding Google Presentation Starting on a Particular Slide
Step 1 - Get your embed code:
<iframe src="https://docs.google.com/presentation/d/1m21r-mzC8CaWw80IjYYvIqKjV9xSXrJ9eOgu47DXZqg/embed?start=false&loop=false&delayms=5000" width="900" height="479" frameborder="0" allowfullscreen="allowfullscreen"></iframe>
Step 2 - Add # and the number of the slide you want display first to the end of the doc's url:
<iframe src="https://docs.google.com/presentation/d/1m21r-mzC8CaWw80IjYYvIqKjV9xSXrJ9eOgu47DXZqg/embed?start=false&loop=false&delayms=5000#3" width="900" height="479" frameborder="0" allowfullscreen="allowfullscreen"></iframe>