What property is 12 (x+4) = 12x+48

This is the distributive property.

Step-by-step explanation:

Notice how they are multiplying the twelve by both numbers in the parenthesis, this is known as distributing. Whenever you multiply a number and a variable you usually just connect them, except for 12 and 4, they must be multiplied together as they are like terms.

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Distributive property