How do I figure out the grade of a driveway?

Let’s say the driveway is 30 feet long and the difference in height from the beginning of the driveway and the end just before the garage is 3 ft. What is the slope or grade? What happens if 10 more feet are added to the driveway (house is further back on the property).

I’m just trying to understand the formula so I can plug in varying numbers.

Also, if a pick-up truck with a trailer behind needs to use the driveway, how do I figure out how steep a driveway can be and it’s ok, that it won’t hit if I back in? The wheels are more than a few feet from the end of the trailer.