Open Question: Newton's Second and Third Law Derivations?

Start with Newton's laws F = ma and F_12 = F_21
Show that if two bodies exert a force on each other, ?p_1 = -?p_2

How do I do this?

Also, I think I need to somehow derive the following equation from those:

?P = (m_A * v_fA + m_B * v_fB) - (m_A * v_iA + m_B * v_iB)

Where:
m_A = mass of cart A
m_B = mass of cart B
v_fA = final velocity of cart A
v_fB = final velocity of cart B
v_iA = initial velocity of cart A
v_iB = initial velocity of cart B

Context: This is for a physics lab report due today. In the lab we took two carts (A and B) and pushed them into each other and gathered the initial and final velocities of both carts. We are trying to determine whether or not momentum is conserved.

Any help you could provide will be greatly appreciated. Also if you have any tips for writing this lab report, please include those. Thank you so much!

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