We can't see your whole code: just a fraction of it.
But ... if you are meant to take an array and get the average then you need to pass two pieces of information into your function: the array itself, and the number of items in it.
Then it's trivial:
int Average(int data[], int len)
{
int sum = 0;
for (int i = 0; i < len; i++)
{
sum += data[i];
}
return sum / len;
}