title: Haskell: Sort by height in descending order
body: <p>I have a list of list as:</p>
  
  <p><code>[[4,7,9],[7,4,9],[9,4,7],[5,8,9],[8,5,9],[9,5,8]]</code></p>
  
  <p>I want to reverse sort them based on base area (3rd side)</p>
  
  <p>eg. <code>sort [[4,7,9],[5,8,9]]</code> should yield <code>[[5,8,9],[4,7,9]]</code>
  since 4+7=11 &lt; 5+8=13</p>
  
  <p>I have implemented the following function:</p>
  
  <p><code>quicksort (s:xs) = quicksort[x | x &lt;- xs, x!!0+x!!1 &lt; s!!2] ++ [s] ++ quicksort[x | x &lt;- xs, x!!0+x!!1 &gt;= s!!2]</code></p>
  
  <p>But it doesn't give the correct output.</p>
  
  <p>Any help would be appreciated.</p>