if (Serial.available() > 0) {

    int inByte = Serial.read();

    switch (inByte) {

    case 'a':    

      digitalWrite(2, HIGH);

      delay(15);

      digitalWrite(2, LOW);

      delay(15);

      digitalWrite(3, HIGH);

      delay(15);

      digitalWrite(3, LOW);

      delay(15);

      Serial.println("Pin's 2 & 3 are blinking really fucking fast");

      break;
}