Use colspan=’2′:

<table border="1">
    <thead>
        <tr>
            <td></td>
            <td colspan="2"><b>Language</b></td>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td rowspan="2"><b>Semester</b></td>
            <td><b><i>Absences</i></b></td>
            <td><b><i>Grades</i></b></td>
        </tr>
        <tr>
            <td style="border-bottom-color: white">
                <div>1</div>
                <div>2</div>
                <div>3</div>
                <div>4</div>
            </td>
            <td>
                <div>1</div>
                <div>2</div>
                <div>a</div>
            </td>
        </tr>
        <tr>
            <td><b>Master Grade</b></td>
            <td></td>
            <td>10</td>
        </tr>
        <tr>
            <td >Media</td>
            <td colspan="2">10</td>
        </tr>
    </tbody>
</table>

Simply add colspan="2" as an attribute to the td

<table border="1">
    <thead>
        <tr>
            <td></td>
            <td colspan="2"><b>Language</b></td>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td rowspan="2"><b>Semester</b></td>
            <td><b><i>Absences</i></b></td>
            <td><b><i>Grades</i></b></td>
        </tr>
        <tr>
            <td style="border-bottom-color: white">
                <div>1</div>
                <div>2</div>
                <div>3</div>
                <div>4</div>
            </td>
            <td>
                <div>1</div>
                <div>2</div>
                <div>a</div>
            </td>
        </tr>
        <tr>
            <td><b>Master Grade</b></td>
            <td></td>
            <td>10</td>
        </tr>
        <tr>
            <td>Media</td>
            <td colspan="2">10</td>
        </tr>
    </tbody>
</table>

Let’s consider the following table; jsfiddle: https://jsfiddle.net/cgfqu1ap/

<table border="1">
    <thead>
        <tr>
            <td></td>
            <td colspan="2"><b>Language</b></td>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td rowspan="2"><b>Semester</b></td>
            <td><b><i>Absences</i></b></td>
            <td><b><i>Grades</i></b></td>
        </tr>
        <tr>
            <td style="border-bottom-color: white">
                <div>1</div>
                <div>2</div>
                <div>3</div>
                <div>4</div>
            </td>
            <td>
                <div>1</div>
                <div>2</div>
                <div>a</div>
            </td>
        </tr>
        <tr>
            <td><b>Master Grade</b></td>
            <td></td>
            <td>10</td>
        </tr>
        <tr>
            <td>Media</td>
            <td>10</td>
        </tr>
    </tbody>
</table>

How can I merge the last row’s second column with the third column within Language column?