1. **State the problem:** We are given two functions $f(x) = x + 3$ and $g(x) = x - 1$. We need to find the function $(f - g)(x)$ and simplify it. 2. **Formula used:** The difference of two functions is defined as: $$ (f - g)(x) = f(x) - g(x) $$ 3. **Apply the formula:** Substitute the given functions: $$ (f - g)(x) = (x + 3) - (x - 1) $$ 4. **Simplify the expression:** $$ (f - g)(x) = x + 3 - x + 1 = (x - x) + (3 + 1) = 0 + 4 = 4 $$ 5. **Conclusion:** The simplified form of $(f - g)(x)$ is $4$. This means the difference between the two functions is a constant function equal to 4 for all $x$.