What substitution really means
Replace every letter with its value, keep the operations, and follow BIDMAS.
Substitution means swapping a letter in an expression or formula for a known number, then working out the value. The expression itself does not change β only the letters get replaced.
If , then becomes . Notice that means "3 multiplied by " β the multiplication sign is usually hidden, but it is still there once you substitute.
A small habit that pays off: always use brackets when you substitute a negative number. If , then becomes . Without the brackets it is easy to write , which is wrong. Brackets keep the sign attached to the right number.
- Substitute means swap each letter for its given number.
- Hidden operations come back when you write the multiplication.
- Bracket negative values to keep their signs safe.
- Apply BIDMAS once everything is numerical.