table='''<table>
    <tr>
        <td>Num of something 1</td>
        <td>4532</td>
        <td>78</td>
    </tr>
    <tr>
        <td>Num of something 2</td>
        <td>0</td>
        <td>235</td>
    </tr>
    <tr>
        <td>Num of something 3</td>
        <td>8</td>
        <td>6589</td>
    </tr>
</table>'''
# если найден Num of something 3 то вывести 6589
if root.xpath(".//td[text()="+search_string+"]"):
    result = item.xpath('//td[position()=3]')