3 (x+y) = 3x+3y is what property

the answer is distributive property

Step-by-step explanation:

the distributive property tell us that the multiplication of a number (in this case number 3) by a sum (in this case x+y) is equal to the sum of the multiplications of that number by each of the terms belonging to the sum:

3 (x+y) = 3*x+3*y

3 (x+y) = 3x+3y