Answer
The momentum of the propellant gases is $0.866~kg~m/s$.
Work Step by Step
Let $p_p$ be the momentum of the propellant gases.
$p_r+p_b+p_p = 0$
$p_p = -m_bv_b-m_rv_r$
$p_p = -(0.00720~kg)(601~m/s-1.85~m/s)-(2.80~kg)(-1.85~m/s)$
$p_p = 0.866~kg~m/s$
The momentum of the propellant gases is $0.866~kg~m/s$.
