const int len = 10;
int a[len] = {0, 2, -4, 12, -52, 42, -96, 7, -23, 99};

void setup() {

  // Declare a local variable to store our average
  float avg = 0;

  // Use a for loop to step through each element, adding it to avg
  for ( int i = 0; i < len; i++ ) {
    avg += a[i];
  }

  // Divide by the length of the array to determine the average
  avg /= len;

  // Initialize the serial port and print the answer
  Serial.begin(9600);
  Serial.println(avg, 4);
}

void loop() {
  // put your main code here, to run repeatedly:
}

Simulator: https://tinkercad.com/things/iNTKSebDvIw