NEW TO ARDUINO #gasPin was not declared in this scope

Using Arduino Programming Questions
1

How should I modify the program ?

sketch_feb20a.ino (112 Bytes)

2 
void setup()
{
  Serial.begin(9600);
}

void loop() {
  Serial.println(analogRead(gasPin));
  delay(1000);
}

Define "gasPin" somewhere? like "static const gasPin = 1; // gas sensor on analog pin 1"