Okay, its actually pretty simple.
3v-9v
All we need to pay attention to is the 3 and the 9. It may actually be better if you write this as 3v + (-9v) just so it makes more sense.
If we add 3 to -9, we will get -6. So we have -6v = 30
We need to get v by itself to solve for it so if we were to divide both sides by -6, we would get v = -5
We can check this by plugging v into the first equation that you were given.
