Question

how to bring back the extincted animals

Answer 1

there is no way we can get the animals which have gone extinct because if there are no animals of a particular type then it impossible to get a young one of that specie.since interbreeding process is not possible. all we can do is that we can protect the animals which are there at present.

Answer 2

it is not possible to bring back the extinct animals as they are no more for interbreeding. but it might be possible in the future by the invention of time machines. Or there can be chances of cloning of the animals as it was done in the jurassic  park movie. though it seems hypothetical but in the future cloning of animals might be possible.