Subtract $ \dfrac{x^2}{2} $ from $ 2 $ to get $ \dfrac{ \color{purple}{ -x^2+4 } }{ 2 }$.

Step 1: Write $ 2 $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $\color{blue}{ 2 }$.

$$ \begin{aligned} 2- \frac{x^2}{2} & \xlongequal{\text{Step 1}} \frac{2}{\color{red}{1}} - \frac{x^2}{2} = \frac{ 2 \cdot \color{blue}{ 2 }}{ 1 \cdot \color{blue}{ 2 }} - \frac{ x^2 }{ 2 } = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ 4 } }{ 2 } - \frac{ \color{purple}{ x^2 } }{ 2 }=\frac{ \color{purple}{ -x^2+4 } }{ 2 } \end{aligned} $$