"The sum of the momentums before the hammer was thrown."
v_a=0
m_a=70 kg
v_h=0
m_h=4kg
P_b=m_a*v_a+m_h*v_h
P_b=70*0+4*0=0
"The sum of the momentums after the hammer was thrown."
m_a=70kg
v_a=3 ms^-1
m_h=4kg
v_h=v=?
P_a=m_a*v_a+m_h*v_h
P_a=70*3+4v
P_a=210+4v
"Since the momentum is conserved, we can write the following equation."
P_b=P_a
0=210+4v
-210=4v
v=-210/4
v=-52.5 ms^-1" should be thrown in the opposite direction."