The Rainbow Rule.
Multiplying a value outside the parentheses to EVERY term inside the parentheses.
A fundamental axiom of real numbers, essential for polynomial multiplication.
Draw arrows (rainbows) from the outside number to each inside number.
In Plain English: Imagine you are a waiter. You have to give water to EVERYONE at the table (inside the parentheses), not just the first person. You 'distribute' the water.
In The Real World: Buying Combo Meals. If you buy 3 'Burger+Fry' combos, you have 3 Burgers AND 3 Fries. 3(B+F) = 3B + 3F.
Forgetting the second number. Students often write 2(x+3) as 2x + 3. Wrong! It must be 2x + 6.