Arduino bluetooth car

char t;
 
void setup() {
pinMode(3,OUTPUT);   //left motors forward
pinMode(4,OUTPUT);   //left motors reverse
pinMode(5,OUTPUT);   //right motors forward
pinMode(6,OUTPUT);   //right motors reverse
pinMode(13, OUTPUT);
pinMode(12, OUTPUT);
pinMode(11, OUTPUT);
Serial.begin(9600);
 
}
 
void loop() {
if(Serial.available()){
  t = Serial.read();
  Serial.println(t);
}
 
if(t == 'F'){            //move forward(all motors rotate in forward direction)
  digitalWrite(6,HIGH);
  digitalWrite(4,HIGH);
}
 
else if(t == 'B'){      //move reverse (all motors rotate in reverse direction)
  digitalWrite(5,HIGH);
  digitalWrite(3,HIGH);
}
 
else if(t == 'L'){      //turn right (left side motors rotate in forward direction, right side motors doesn't rotate)
  digitalWrite(4,HIGH);
}
 
else if(t == 'R'){      //turn left (right side motors rotate in forward direction, left side motors doesn't rotate)
  digitalWrite(6,HIGH);
}
if(t == 'W'){
  digitalWrite(13, HIGH);
}

if(t == 'w'){
  digitalWrite(13, LOW);
}

if(t == 'U'){
  digitalWrite(12, HIGH);
  digitalWrite(11, HIGH);
}

if(t == 'u'){
  digitalWrite(12, LOW);
  digitalWrite(11, LOW);
}

delay(100);
}