Obstacle Avoider Robo

#include <Servo.h>
Servo myservo;
int triggerPin = 11;
int echoPin = 7;
int revleft4 = 2;
int fwdleft5 = 4;
int revright6 = 5;
int fwdright7 = 6;
float distance;
float duration;
float distancelt;
float distancert;
void setup() {
pinMode(revleft4, OUTPUT);
pinMode(fwdleft5, OUTPUT);
pinMode(revright6, OUTPUT);
pinMode(fwdright7, OUTPUT);
pinMode(triggerPin, OUTPUT);
pinMode(echoPin, INPUT);
Serial.begin(9600);
myservo.write(90);
myservo.attach(3);
}
void goesforward() {
  Serial.println("Going Forward");
  digitalWrite(fwdleft5, HIGH);
  digitalWrite(fwdright7, HIGH);
}
void goesbackward() {
  Serial.println("Going Backward");
  digitalWrite(revleft4, HIGH);
  digitalWrite(revright6, HIGH);
  delay(500);
  digitalWrite(revleft4, LOW);
  digitalWrite(revright6, LOW);
  Serial.println("Going Backward Stopped");
}
void turnright() {
  Serial.println("Turning Right");
  digitalWrite(fwdleft5, HIGH);
  digitalWrite(revright6, HIGH);
  delay(500);
  Serial.println("Turning Right Stopped");
  digitalWrite(fwdleft5, LOW);
  digitalWrite(revright6, LOW);
}
void turnleft() {
  Serial.println("Turning Left");
  digitalWrite(fwdright7, HIGH);
  digitalWrite(revright6, HIGH);
  delay(500);
  Serial.println("Turning Left Stopped");
  digitalWrite(fwdright7, LOW);
  digitalWrite(revright6, LOW);
}
void checkdistance() {
  digitalWrite(triggerPin, 0);
  delayMicroseconds(2);
  digitalWrite(triggerPin, 1);
  delayMicroseconds(10);
  duration = pulseIn(echoPin, 1);
}
void loop() {
  checkdistance();
  distance = duration / 58.2;
if (distance >=15.00) {
  goesforward();
}
else if (distance = 0) {
  Serial.println("Please input the Ultrasonic Sound Sensor");
}
else {
  myservo.write(180);
  delay(400);
  checkdistance();
  distancelt = duration / 58.2;
  delay(400);
  myservo.write(90);
  delay(400);
  myservo.write(0);
  checkdistance();
  distancert = duration / 58.2;
  delay(400);
  myservo.write(90);
  delay(250);
  if (distancert >= distancelt) {
    goesbackward();
    turnright();
  }
  else {
    goesbackward();
    turnleft();
  }
}
}