def z_score(X, mu, std):
    """
    Applies z_score normalization using previously computed mu and sigma.

    Parameters:
        x (np.array)[n, k]: The un-normalized data.
        mu (np.array)[k]: The previously computed mean vector.
        sigma (np.array)[k]: The previously computed standard deviations.

    Returns:
        x' (np.array)[n, k]: The normalized data.
    """
    return (X - mu) / std