<?php

$media_de_aproveitamento = 0;

if(isset($_POST['N1'])) {
    $n1 = $_POST['N1'];
}
if(isset($_POST['N2'])) {
    $n2 = $_POST['N2'];
}
if(isset($_POST['N3'])) {
    $n3 = $_POST['N3'];
}
if(isset($_POST['MEX'])) {
    $mex = $_POST['MEX'];
}

?>
<form action="#" method="post" id="form1">
  Nota 1:<br>
  <input type="text" name="N1" value="<?php echo $n1 ?>">
  <br>
  Nota 2:<br>
  <input type="text" name="N2" value="<?php echo $n2 ?>">
    <br>
  Nota 3:<br>
  <input type="text" name="N3" value="<?php echo $n3 ?>">
  <br><br>
  Media dos Exercicios:<br>
  <input type="text" name="MEX" value="<?php echo $mex ?>">
  <br><br>
   <input type="submit" value="Enviar">
</form>

Media:  <?php $media_de_aproveitamento = ( $n1 + $n2 * 2 + $n3 * 3 + $mex) / 7 ;
        echo $media_de_aproveitamento; ?>