From Newton's third law of motion, the force that is exerted to the boy must be the same force exerted to the girl only that the direction is opposite. The net force in this item is,
F = m x a
Girl:
F = (37 kg)(2.3 m/s²)
F = 85.1 N
In the boy's case,
F = m x a
a = F/m
Boy:
a = 85.1 N/63 kg
a = 1.35 m/s²
Answer: 1.35 m/s²