Answer
The magnitude of the momentum of the antineutrino is $1.61\times 10^{-22}~kg~m/s$, and it is moving to the left.
Work Step by Step
We can use conservation of momentum to find the momentum of the antineutrino. We can assume that the initial momentum is zero. Let $p_a$ be the momentum of the antineutrino.
$(5.60\times 10^{-22}~kg~m/s)+ (3.50\times 10^{-25}~kg)(-1.14\times 10^{3}~m/s) + p_a = 0$
$p_a = -(5.60\times 10^{-22}~kg~m/s)- (3.50\times 10^{-25}~kg)(-1.14\times 10^{3}~m/s)$
$p_a = -(5.60\times 10^{-22}~kg~m/s)+ (3.99\times 10^{-22}~kg~m/s)$
$p_a = -1.61\times 10^{-22}~kg~m/s$
The magnitude of the momentum of the antineutrino is $1.61\times 10^{-22}~kg~m/s$, and it is moving to the left.
