how to display specific Category if post have more one Category?

Confusingly, get_the_category() returns an array of categories, so you’re going to need to loop through them.

It sounds like you want whichever category is assigned to the current post, and is also a sub-category of category id 11. If that’s the case, use

<?php
// Get all the categories assigned to this post
$categories = get_the_category();
// Loop through the array that was returned
foreach($categories as $category) {
    // If this category is a sub-category of category 11
    if($category->parent == 11) {
        // Set the $parent to it
        $parent = $category;
        // And exit the foreach loop
        break;
    }
}
?>