Well, now I understand better what you want. Since I don't have your classes "mat gen" and "eqnnum" your results could be different from mine but try this code:
Code:
<body>
<table>
<tr id="a0000000013">
<td style="width:40%"> </td>
<td style="vertical-align:middle; text-align:right"><img alt="$\displaystyle \delta $" class="math gen" src="../Images/img-0004.png" style="vertical-align: middle; width: auto; height: 1.2em" /></td>
<td style="vertical-align:middle; text-align:left"><img alt="$\displaystyle = a + x_{x_ y} + a + x_{x_ y} + a + x_{x_ y} + a + x_{x_ y} $" class="math gen" src="../Images/img-0005.png" style="vertical-align: middle; width: auto; height: 1.2em" /></td>
<td style="width:40%"> </td>
<td class="eqnnum" style="width:20%"><span>(<span>1.1</span>)</span></td>
</tr>
</table>
<p>where <img alt="$\delta $" class="math gen" src="../Images/img-0004.png" style="width:auto; height: 1em" /> is the ineffective length.</p>
</body>
As you can see, I'm using a "height: 1.2em" and a "widht: auto" for the images in cells and a "height: 1em" and a "width: auto" for the inline image. Without your classes, the .mobi looks fine for all models of Kindle Fire (at least, in Kindle Previewer). I attach the respective epub and mobi files so you can check the results.
Regards
EDIT: By the way, you don't need to employ an inline image for the delta symbol. Instead you could use:
Code:
<p>where <span style="font-family: Symbol; font-size: 1.2em">d</span> is the ineffective length.</p>