Problema com Editar formulario

[Dorgas21 0]

<?php

include "connection.php";

$id = $_GET['id'];

$busca = mysqli_query("SELECT * FROM comentarios_tb WHERE id = $id");
$row = mysqli_fetch_assoc($busca);

?>
<form method="POST" action="salvar.php">

<input type="text" value="<?php echo $row['campo']; ?>" name="campo" />
<input type="hidden" value="<?php echo $id; ?>" name="id" />

<input type="submit" value="editar" />

</form>

<?php

$campo = $_POST['campo'];
$id = $_POST['id'];

mysqli_query($conexao,"UPDATE comentarios_tb SET campo = '$campo' WHERE id = $id");
print "Editado com sucesso";

?>

ERROS :

Warning: mysqli_query() expects at least 2 parameters, 1 given in C:\Program Files (x86)\VertrigoServ\www\Php\Cadastro\alter.php on line 7

Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, null given in C:\Program Files (x86)\VertrigoServ\www\Php\Cadastro\alter.php on line 8

Estou com duvida sobre estes dois erros não estou conseguindo achar o error.

[DavidL 1]

https://support.google.com/docs/troubleshooter/2853025?hl=es