sum them all up and divide by the size of the array. For example

What is not clear to me is: does the array have only positive numbers or are you finding the average of only positive numbers?

If the first, then Ancient Dragon has the con
If the second, you'd need to pull out only the positive numbers and divide by their count.

Perhaps ...
int a[6] = {2,-3, 4, -5, 6, -7};

int b[6] = new int[];
loop through a, placing positive values in b
then ...
The sum is 2+4+6 = 12
The average is 12/3 = 4

Think about how you add up. Suppose I ask you to add the
number in the series 2,6,9. you don't go let me put the numbers into
an array. you simply keep a running total,

So in your case

start a total
total=0
then IF the number is >0 add to total
divide total by the number of >0 numbers (or 42 ;-)

Just because it is programming it doesn't mean that you can forget you kindergarden maths.