Question

why in case of bullet amd gun we take the velocity of gun or bullet negative

Answer 1

we take the recoil velocity as negative because it is the force acting against gravity.

Answer 2

Let me help you out..
In physics we use sign convention generally left is taken as positive and right as negative.
It is upto you which one you are convinent in....as i take negative as left and positive as right.
And one more point the answer of the problem will remain the same in both cases but the convention has to be used properly..
HOPE IT HELPS...