Sure you can divide by zero. Get 5 apples, divide them amongst 0 people and see with how many apples you are left with. Also see if the universe ends ("cannot" is such a scary word for mathematicians).
The reason we are forced to say that such a division is impossible is because otherwise the equation 1*0 = 0 would become absurd. If 1*0 = 0 then 0/0 will have to be = 1. So we take the easy way out and instead of pursuing the unknown correct answer we simply say 'you can't divide by zero' even though it's just not true.
In the OPs question, the answer is indeed 0 if operator precedency is not taken into account and - 30 if it is. Simply assuming you must use operator precedency is plain stupid as it was never mentioned as a condition. And in a real world situation if you get 6 apples then another 6 multiplied by 6 and take out 6 you end up with indeed 30 apples, not -30 and not 0.
So it is all about context and the more exact conditions you set, the more useful the answer becomes. This comes from a coder