title: iOS - How to use NSExpression with one unknown parameter
body: <p>I'm using <code>NSExpression</code> to evaluate a formula in a string.</p>
  
  <p>Example -</p>
  
  <pre><code>NSString *formula = @"7+11";
  NSExpression *exp = [NSExpression expressionWithFormat: formula];
  NSNumber *expResult = [exp expressionValueWithObject:nil context:nil];
  </code></pre>
  
  <p>Everything works just fine but...</p>
  
  <p>What if I have the following formula "7+x=18"? How can I evaluate this formula and find "x" and get the result 11?</p>