A bus travels 15 blocks to the east. The bus then drives in reverse for 4 blocks back toward the east. Finally, the bus driver puts the bus back in
drive and drives 7 blocks toward the east. What is the bus's final
displacement? What distance did the bus travel?

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s

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hope this !