1. **State the problem:** Calculate the value of the expression $ (a + b) c - d $ given $a=1$, $b=2$, $c=3$, and $d=4$. 2. **Write the formula:** The expression is $ (a + b) c - d $. 3. **Substitute the values:** Replace $a$, $b$, $c$, and $d$ with their given values: $$ (1 + 2) \times 3 - 4 $$ 4. **Perform the operations inside the parentheses first:** $$ 3 \times 3 - 4 $$ 5. **Multiply:** $$ 9 - 4 $$ 6. **Subtract:** $$ 5 $$ **Final answer:** $5$