Answer:
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
