Sharing the multiplier.
a(b + c) = ab + ac.
This is the foundational rule for Algebra.
Multiply the outside number by EVERY inside number.
Plain English: If you buy 3 combo meals, that means you bought 3 burgers AND 3 fries. You distribute the '3' to everything inside the bag.
Real-world example: Mental math. 5 x 104 is hard. 5 x (100 + 4) is easy. 500 + 20 = 520.
Only multiplying the first number. 3(x+2) is 3x+6, NOT 3x+2.