ction $y=f(x)=x^2$.
    <ol>
      <li>The derivative is $f'(x) = ?$. At the point $x=3$ the
      derivative is $f'(3)=?$ and the output $y$ is
      $y=f(3)=?$.</li>

      <li>If we move from the point $(3,f(3))$ to the point $(x,y)$
      along the tangent line, then a small change in $x$ is
      $dx=x-3$. What is $dy$?</li>

      <li>Differential notation states that a change in the output
      $dy$ equals the derivative times a change in the input $dx$,
      which gives us the equation $dy=f'(3)dx