def _grade_level(total_sentences, total_words, total_syllables):
    """
    Flesch-Kincaid Grade Level Score.

    Parameters
    ----------
        total_sentences: int, number of sentences.
        total_words: int, number of words.
        total_syllables: int, number of syllables.

    Returns
    -------
        grade_level_score.
    """
    grade_level_score = (0.39 * (total_words / total_sentences) + 11.8 * (total_syllables / total_words)) - 15.59
    return grade_level_score