To infinity and beyond that

How can $\infty - \infty = 0$ ? Rather to say infinity is not a number and just an assumption for ending of numbers. So any algebra laws are not valid for infinity. So the answer should be undefined.

As the author of this question mentions in his solution that if we take out 2 from infinity the infinity is not effected and remains same. So, $\infty - \infty = 2$ . But this condition holds true even for any number because if you take any number from infinity the result will always be infinity. So 3,4,5,6,.... can be the answers for this question. As a single question has infinity answers the actual answer is undefined.