Take into account that the momentum p is given by the following formula:

where,
p: momentum of the bat = 0.86 kg*m/s
m: mass = ?
v: speed of the bat = 2.9m/s
Solve the previous equation for m, replace the values of v and p, and simplify:
![m=(p)/(v)=\frac{0.86\operatorname{kg}\cdot(m)/(s)}{2.9(m)/(s)}\approx0.30\operatorname{kg}]()
Hence, the mass of the bat is approximately 0.30kg