Step-by-step explanation:
I won't give you the answers. It won't help you in the long run. Instead, let's look at it logically:
You have two of something and you're gonna turn it into something else. That thing doesn't just disappear, it has to stay there. You can't act as if there's only one of them in the end result, that doesn't make sense.
Balancing chemical equations is about finding the ratios at which things occur. In the first example,
N2 + F2 -> NF3
You have two N on the left, one on the right. Better double the one on the right so you have equal amounts.
N2 + F2 -> 2NF3
Now, since there are two Ns on each side, there must be 6 Fs on the right (2×3). You only have 2 on the left side, so the next question is: how do I get from 2 to 6?
Simple! Multiply by 3:
N2 + 3F2 -> 2NF3
Let's check left side vs right side to see if we're right:
Left:
2 N
6 F (3×2)
Right:
2 N
6 F (2×3)
Left side = Right side, so that's the right answer.
Don't let questions like these stress you out. Just balance out the numbers so that there's the same amount of stuff on each side.