I am going over the practice units but I am stuck on Unit 1, question 12. I don't know how to do this? Or why I would be doing this? please help!

Hi Emily

Thank you for your question, and apologies for the delay in response.

The reason you would create a second (or duplicate) slide master is so that you have two different template designs to use within one presentation.

Once you have created two master slide designs, you need to preserve both of them (each should have a pin symbol next to them if they are preserved, this can be done by right-clicking on top of a slide and selecting Preserve master).

Then exit the Master slide view. Use the Design templates task pane to see your two template designs. Make sure the new slide you have created in showing on your screen, then use the dropdown arrow next to the design you want to apply to this slide, and choose the Apply to selected slides option.

I hope this helps.
Amanda