Display:

$\begin{array}{}\text{(2.1)}& E=m\⁢c^2\end{array}$

Source:

<mtable>
     <mlabeledtr id="e-is-m-c-square">
          <mtd>
               <mtext> (2.1) </mtext>
          </mtd>
          <mtd>
               <mrow>
                    <mi>E</mi>
                    <mo>=</mo>
                    <mrow>
                         <mi>m</mi>
                         <mo>&it;</mo>
                         <msup>
                              <mi>c</mi>
                              <mn>2</mn>
                         </msup>
                    </mrow>
               </mrow>
          </mtd>
     </mlabeledtr>
</mtable>