python lxml как доделать

table='''<table>
    <tr>
        <td>ccc</td>
        <td>12</td>
        <td>14</td>
    </tr>
    <tr>
        <td>bbb</td>
        <td>16</td>
        <td>18</td>
    </tr>
    <tr>
        <td>aaa</td>
        <td>20</td>
        <td>22</td>
    </tr>
</table>'''

item = root.xpath('.//td[text()="'+string_parameter+'"]'):
result = item.xpath('//td[position()=3]')