$( ".left_arr" ).click(function() {
  if(parseInt($( ".line" ).css("margin-left")) > (($( ".line .slide" ).length-1)*-400)) {
    $( ".fbline" ).animate({
    "margin-left": "-=400",
    }, 1000, function() {
    });
  }
});
$( ".right_arr" ).click(function() {
  if(parseInt($( ".line" ).css("margin-left"))<0) {
    $( ".line" ).animate({
    "margin-left": "+=400",
    }, 1000, function() {
    });
  }
});

<div class="container">
  <div class="line">
    <div class="slide">1</div>
    <div class="slide">2</div>
    <div class="slide">3</div>
  </div>
</div>

.slide{
  width: 300px;
  padding: 0 50px;
  display: inline-block;
  text-align: center;
}
.container {
  margin: 0 80px;
  overflow: hidden;
}
.line {
  display: inline-block;
  width: 9999px;
}