There is a property in math called the "Distributive Property". The distributive property lets us multiply a sum by multiplying each addend separately and then add the products.
In this question, 3 is the multiplier and 12 and b are the addends. Thus, as we can see from the right hand side, 3 multiplies 12 and b separately and then adds the products. Thus, this question represents the Distributive Property.