Subtract $ \dfrac{1}{x^2} $ from $ \dfrac{1}{36} $ to get $ \dfrac{ \color{purple}{ x^2-36 } }{ 36x^2 }$.

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}{ x^2 }$ and the second by $\color{blue}{ 36 }$.

$$ \begin{aligned} \frac{1}{36} - \frac{1}{x^2} & = \frac{ 1 \cdot \color{blue}{ x^2 }}{ 36 \cdot \color{blue}{ x^2 }} - \frac{ 1 \cdot \color{blue}{ 36 }}{ x^2 \cdot \color{blue}{ 36 }} = \\[1ex] &=\frac{ \color{purple}{ x^2 } }{ 36x^2 } - \frac{ \color{purple}{ 36 } }{ 36x^2 }=\frac{ \color{purple}{ x^2-36 } }{ 36x^2 } \end{aligned} $$