1. **State the problem:** Simplify the expression $x + 3x - 6$. 2. **Combine like terms:** The terms $x$ and $3x$ are like terms because they both contain the variable $x$. 3. **Add the coefficients:** $$x + 3x = (1 + 3)x = 4x$$ 4. **Rewrite the expression:** $$4x - 6$$ 5. **Final answer:** The simplified form of the expression is $4x - 6$. This means you add the coefficients of the $x$ terms and keep the constant term as is.