def get_op_by_tx(block, tx_id):
    """Return the operation data by block num and tx id

    :param block (int): Block number
    :param tx_id (str): Transaction ID
    :return (dict): Operation data
    """
    for transaction in block['transactions']:
        if transaction['transaction_id'] == tx_id:
            return transaction['operations'][0][1]
    raise ValueError('Invalid transaction ID')
