Answer
$p = 3.0~kg~m/s~~$ to the north
Work Step by Step
Let north be the positive direction. We can find the total momentum of the system:
$p = m_1v_1+m_2v_2+m_3v_3$
$p = (3.0~kg)(3.0~m/s)+(4.0~kg)(-5.0~m/s)+(7.0~kg)(2.0~m/s)$
$p = 3.0~kg~m/s~~$ to the north