Evaluating Expressions by Substitution (Grade 6)
To evaluate an expression, replace each letter with its given value and then do the arithmetic, following the order of operations.
Understanding evaluating expressions by substitution
If n = 4, then 2n + 3 means 2·4 + 3 = 11. Substitution is the bridge between a general rule (with letters) and a specific answer (with numbers) — and it's how you check a solution.
Seeing it in action
Worked example
Evaluate 3a + 2 when a = 5.
Replace a with 5: 3·5 + 2.
Multiply first: 15 + 2 = 17.
Substitute first, then compute.
A common mistake, and the fix
Writing the number next to the letter instead of multiplying.
3a with a = 5 means 3 × 5 = 15, not 35, so 3a + 2 = 17. Then keep to the order of operations: multiply before adding.
Try a few
Evaluate x − 6 when x = 10
Evaluate 2(b + 1) when b = 3
b + 1 = 4 first, then 2 × 4.
Evaluate 4m when m = 7
Function Foundry
A calm pre-algebra game for function machines, inverse operations, and solving for an unknown.
Try it in Numeris.
A short lesson, then practice with feedback on every answer.
Put the reasoning on paper.
Try six fraction, angle, perimeter and area questions with explained answers, for grades 4–6.
Free printable · 3 pagesText version (screen-reader friendly): Draw it. Explain it. Math sample
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