`
$
\eqalign{
&k(x)=\frac{{x^3-4}}
{{3x}}\cr
&k(x)=\frac{{x^3}}
{{3x}}-\frac{4}
{{3x}}\cr
&k(x)=\frac{1}
{3}x^2-\frac{4}
{3}x^{-1}\cr
&k\,'(x)=\frac{1}
{3}\cdot2x-\frac{4}
{3}\cdot-1\cdot x^{-2}\cr
&k\,'(x)=\frac{2}
{3}x+\frac{4}
{3}\cdot\frac{1}
{{x{}^2}}\cr
&k\,'(x)=\frac{2}
{3}x+\frac{4}
{{3x{}^2}}\cr}$
$\eqalign{
&k\,'(x)=\frac{2}
{3}x\cdot\frac{{x^2}}
{{x^2}}+\frac{4}
{{3x^2}}\cr
&k\,'(x)=\frac{{2x^3}}
{{3x^2}}+\frac{4}
{{3x^2}}\cr
&k\,'(x)=\frac{{2x^3+4}}
{{3x^2}}\cr}
$