If I run 100 yards in 15 seconds, what is my average speed

Average speed is the distance covered in a certain amount of time. So, the equation to figure this out is (distance) divided by (time).

However, if you're using SI units, which you likely are if you're in physics, you'll need to convert yards to meters.
If you convert, you should get m/s as your average speed units.
If you don't convert, you should get yds/s as you average speed units.