title: LaTeX: Numbering only last line of multi-line equation in \align
body: <p>I want to number and make a reference to just the last line of a multi-line equation in <code>\align</code>. Here is an example:</p>
  
  <pre><code>\begin{align}
  z_1^2 &amp;= \left( 2 e^{i \frac{\pi}{3}} \right) \left( 2 e^{i \frac{\pi}{3}} \right)\\
  &amp;= 4 e^{i \frac{2 \pi}{3}}
  \end{align}
  </code></pre>
  
  <p>I only want to number the <code>&amp;= 4 e^{i \frac{2 \pi}{3}}</code> part, rather than both lines of the <code>\align</code>. How do I do this?</p>