CODE:-
int first=9;  
int second=10;
int third=11;
int forth=12;
String inval;
int wait=7000;
void setup() {
  // put your setup code here, to run once:
pinMode(first,OUTPUT);
pinMode(second,OUTPUT);
pinMode(third,OUTPUT);
pinMode(forth,OUTPUT);
Serial.begin(9600);
    digitalWrite(first,HIGH);
  digitalWrite(second,HIGH);
  digitalWrite(third,HIGH);
  digitalWrite(forth,HIGH);
}

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

if (Serial.available()==1){
  inval=Serial.readString();
  Serial.println(inval);  
}

inval=Serial.readString();
 Serial.println(inval);  
if (inval=="launch first rocket") {
  digitalWrite(first,LOW);
  inval=Serial.readString();
  delay(wait);
  digitalWrite(first,HIGH);
}
if (inval=="launch second rocket") {
  digitalWrite(second,LOW);
  inval=Serial.readString();
  delay(wait);
  digitalWrite(second,HIGH);
}
if (inval=="launch third rocket") {
  digitalWrite(third,LOW);
  inval=Serial.readString();
  delay(wait);
  digitalWrite(third,HIGH);
}
if (inval=="launch forth rocket") {
  digitalWrite(forth,LOW);
  inval=Serial.readString();
  delay(wait);`
  digitalWrite(forth,HIGH);
}
if (inval=="launch all") {
  digitalWrite(first,LOW);
  digitalWrite(second,LOW);
  digitalWrite(third,LOW);
  digitalWrite(forth,LOW);
  inval=Serial.readString();
  delay(wait);
    digitalWrite(first,HIGH);
  digitalWrite(second,HIGH);
  digitalWrite(third,HIGH);
  digitalWrite(forth,HIGH);
}
}