Question

write the law of conservation of momentum & write it's mathematical expression .

Answer 1

law of conservation of mass states that the momentum of the system remains constant if no external force is acting on it
mathematically
m1u1+m2u2=m1v1+m2v2

Answer 2

The law of conservation of momentum states that
the total momentum of a closed system does not change.

This means that when two objects collide
the total momentum of the objects before the collision
is the same as
the total momentum of the objects after the collision.
Total momentum means
the momentum of object 1 plus the momentum of object 2.

Object 1 might not have the same momentum before and after
the collision
but the amount of momentum object 1 gains from the collision
is the same as the amount of momentum object 2 loses
so the total momentum
remains the same whether the collision is elastic or inelastic.

The total momentum is also the same before and after an explosion.

Mathematically; m1u1+m2u2=m1v1+m2v2