🧱 See It
An expression like 3x + 2 is a recipe with a blank spot. "Substitution" just means filling that blank with a specific number, then following the recipe like normal:
x
→
5
Wherever x was standing, drop in its number instead.
✏️ Draw It
Evaluate 3x + 2 when x = 5:
| Step | Expression |
|---|---|
| Start | 3x + 2 |
| Substitute (bracket it) | 3(5) + 2 |
| Multiply first | 15 + 2 |
| Add | 17 |
🔢 Write It
3x + 2, where x = 5 ⇒ 3(5) + 2 = 17
Always wrap the substituted number in brackets — especially if it's negative. Brackets keep the sign from getting lost before you multiply.
💡 Worked Examples
Example 1: 3x − 4, where x = 6
- Substitute:
3(6) − 4 - Multiply:
18 − 4 - Answer:
14
Example 2: 2x + 3y, where x = −2 and y = 5
- Substitute both, brackets around the negative:
2(−2) + 3(5) - Multiply each part:
−4 + 15 - Answer:
11
🧠 Quick Reference
Replace every letter with its number FIRST, in brackets. THEN follow BEDMAS like normal. Never calculate and substitute at the same time.