Let's use the distributive property to simplify this problem.
The first thing we want to do is change the minus to plus a negative.
Now distribute or multiply the 2 by each of the terms inside the parentheses.
So we get 2(3y) + 2(-4) and this simplifies to 6y + -8
Note that if you wrote your answer as 6y - 8, it means the same thing.