A0.15-kilogram baseball is pitched horizontally at +42 meters per second and is hit by a bat, causing it to move horizontally at -42 m/s. what was the magnitude of the ball's change in momentum? a. 0 kg-m/s b. 6.3 kg-m/s c. 12.6 kg-m/s d. 560 kg-ms

So momentum is 0.15× (42--42)

so; 0.15× 84 = 12.6

so your answer is C !!

for any info ; comment !!

Change in Momentum is a vector so the setup would be delta(p)=mv. And in this situation that means we have

p2=m2*v2 = .15kg * -42m/s = -6.3 kg*m/s
p1=m1*v1 = .15kg * 42m/s = 6.3kg*m/s

So p2-p1= -6.3-6.3= -12.6kg*m/s
So the answer is C 
The reason it is positive is because they were looking for magnitude only which negates the negative sign. Otherwise the negative sign only means moving in opposite direction from original direction since momentum is a vector which includes magnitude and direction.

Hope this helps you understand the concept and how to solve! Any questions please ask