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