what is 0 divide by 0?

Troll answer: dividing my 0 just blew up a city.

Real answer: ditto @ geno

0/0  is an "indeterminate" expression, meaning that it has no fixed value.

Depending upon the problem that created the value 0/0, the answer can be any number ...

it might be 3, or -6, or 0, or -√2, or π, or any other number. It can also could be ∞ or -∞.

Take calculus, and this will be explained to you ...