Multiply $ \dfrac{1}{6k^2} $ by $ \dfrac{1}{3k^2} $ to get $ \dfrac{ 1 }{ 18k^4 } $.

Step 1: Multiply numerators and denominators.

Step 2: Simplify numerator and denominator.

$$ \begin{aligned} \frac{1}{6k^2} \cdot \frac{1}{3k^2} \xlongequal{\text{Step 1}} \frac{ 1 \cdot 1 }{ 6k^2 \cdot 3k^2 } \xlongequal{\text{Step 2}} \frac{ 1 }{ 18k^4 } \end{aligned} $$

Subtract $ \dfrac{1}{k} $ from $ \dfrac{1}{18k^4} $ to get $ \dfrac{ \color{purple}{ -18k^4+k } }{ 18k^5 }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ k }$ and the second by $\color{blue}{ 18k^4 }$.

$$ \begin{aligned} \frac{1}{18k^4} - \frac{1}{k} & = \frac{ 1 \cdot \color{blue}{ k }}{ 18k^4 \cdot \color{blue}{ k }} - \frac{ 1 \cdot \color{blue}{ 18k^4 }}{ k \cdot \color{blue}{ 18k^4 }} = \\[1ex] &=\frac{ \color{purple}{ k } }{ 18k^5 } - \frac{ \color{purple}{ 18k^4 } }{ 18k^5 }=\frac{ \color{purple}{ -18k^4+k } }{ 18k^5 } \end{aligned} $$