# Plug the number in **Substitution** means replace the letter with a number. If `x = 4` and the expression is `2x + 1`: `2(4) + 1 = 8 + 1 = 9` Use brackets when you plug in, especially for negatives: If `x = −3`, `2x + 1 = 2(−3) + 1 = −6 + 1 = −5` ## Many letters `3a + b` with `a = 2`, `b = 5` → `3(2) + 5 = 11` This is how you **evaluate** an expression. ## Brackets are not optional when the number is negative Write the substitution as a full new line: `2x + 1` and `x = −3` ``` 2(−3) + 1 = −6 + 1 = −5 ``` If you write `2−3+1` you have changed the meaning. ### Order after plugging in Use ordinary arithmetic order: brackets, then multiply/divide, then add/subtract. ### Many letters Replace every letter you know. If a letter is still unknown, leave it — but in this lesson both values will be given.