title: How to convert Parametric equation to Cartesian form
body: <p>I need to convert a plane's equation from Parametric form to Cartesian form.
  For example: </p>
  
  <pre><code>(1, 2, -1) + s(1, -2, 3) + t(1, 2, 3)
  </code></pre>
  
  <p>to:</p>
  
  <pre><code>ax+yb+cz+d=0
  </code></pre>
  
  <p>So basically, my question is: how do I find the a, b, c and d, and what's the logic behind the conversion. </p>