title: Big-O notation finding c and n0
body: <p>I've just been introduced to Big-O notation and I've been given some questions. However I'm confused as to how to determine the value of <code>n0</code>.
  I have to show that <code>3n^3  +20n^2 + 5</code> is O(n^3). So far I have:</p>
  
  <pre><code>3n^3 + 20n^2 + 5 &lt;= cn^3
  
  (3 - c)n^3 + 20n^2 + 5 &lt;= 0
  
  5 &lt;= n^3(c - 3) - 20n^2
  
  5 &lt;= n^2(n(c - 3) - 20)
  </code></pre>
  
  <p>I just don't know what to do from here to find n0 and c. Would someone mind explaining?</p>