Let the big fish’s direction be positive:
m(1)v(1)+m(2)v(2)=m(12)+v(12)
(9.90)(2.00)+(0.10)(-4.00)=(9.90+0.10)v(f)
v(f)=1.94m/s[same direction as big fish]
I believe this is a conservation of momentum question. Because (momentum=mass•velocity), the system, if momentum is conserved, will be (mass of big fish•velocity of big fish+mass of small fish•velocity of small fish=mass of both fish combined+velocity of both fish combined). You add the masses because they are now both travelling as 1 mass. Same with final velocity, they are both one mass so they have to travel together at the same velocity.
