Joe measured the volume of a object 5 times and results of 34.5 ml 34.9ml 34.2ml 33.4 ml and 35.9ml what is the average experimental result

The formula for average, or mean, is: add all data and divide by the amount of terms.
So, you would add
34.5+34.9+34.2+33.4+35.9=172.9
You would then divide 172.9 by 5 because you had 5 numbers.
172.9/5=34.58
34.58 is your average.