A Formula for Impulse
Impulse Has Two Definitions
Impulse is defined as change in momentum.Because impulse is defined as change in momentum, it is equal to final momentum  initial momentum.

For a constant force, impulse is also defined as force multiplied by time.Impulse is also equal to force multiplied by time for a constant force.

Because impulse is equal to both change in momentum and to force multiplied by time, change in momentum is equal to force multiplied by time for a constant force.
In addition, a change in momentum usually involves a change in velocity. So, change in momentum is the same thing as mass multiplied by change in velocity. 