Let’s use the *queue dramatic voice* LAW OF THE CONSERVATION OF MOMENTUM!
m1v1i + m2v2i = m1vf + m2vf
m1v1i - m1vf = m2vf - m2v2i
m1(v1i - vf) = m2(vf - v2i)
m2 = [m1(v1i - Vf)] / (vf - v2i)
m2 = [(1)(5 - -1)] / (-1 - -2)
m2 = 6 / 1
m2 = 6 kg
And that’s your final answer! Please press “Thanks”!