To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $2$ and $4$ is $4$. Multiply $\frac{3}{2}$ times $\frac{2}{2}$.
$$\frac{3\times 2}{4}+\frac{x}{4}$$
Since $\frac{3\times 2}{4}$ and $\frac{x}{4}$ have the same denominator, add them by adding their numerators.
