<script type='text/javascript'>

function calculate(){
  
  
  var text2answer = document.getElementById('Text2').value;
  var answer=$quarity-text2answer;
  
  
  alert( answer);

  $sql= 'UPDATE `receved items` SET Quantity=answer where id=18';
  echo $sql;
  
}

</script>"; 

I'm trying to update mysqli database inside JavaScript I have variable that calculated from textbox and get answer I want that answer from variable to update to mysql database please help