What are easy ways to remember the difference between zero slopes and undefined slopes in algebra?

Starting at home, emily traveled uphill to the hardware store for 606060 minutes at just 666 mph. she then traveled back home along the same path downhill at a speed of 121212 mph. what is her average speed for the entire trip from home to the hardware store and back?

Ijust need these 2 questions answered (the second pic is just confirmation i'm not confident in that answer)

What value is a discontinuity of x squared plus 8 x plus 4, all over x squared minus x minus 6?