title: Subtract a generic list from another
body: <p>I am trying remove a list of firmIDs from one list from another.
  I don't really understand linq but I am pretty sure I need to use it.</p>
  
  <pre><code>List&lt;Firm&gt; firms = GetBusinessDevelopmentFirms(database);
  List&lt;Firm&gt; trackedFirms = GetAllCLIFirmsBeingTrackedByUser();
  
  var result = firms.Contains(i =&gt; trackedFirms.Contains(i.FirmID));
  </code></pre>
  
  <p>The last line doesn't work and the system says "unknown method Contains(?)" even though I have put "using System.Linq;" At the top of the class.</p>
  
  <p>My idea was to remove a list of tracked firms from a list of all firms to find the untracked firms.</p>
  
  <p>I hope this makes sense.</p>