Answer: Distributive property of the multiplication.
Step-by-step explanation:
You did not post Gretchen's expression, but i gues that it is 3*(x + y)
I guess that it is the distributive property of the multiplication.
For 3 numbers, A, B and C, we have that:
A*(B + C) = A*B + A*C
here we have:
3*x + 3*y
We can do the inverse process to the distributive property, and get:
3*x + 3*y = 3*(x + y)
which i guess is Gretchen's expression.
