Mass of the javelin is 16.25 kg.
Explanation for the problem.
Consider the given data:
Unbalanced force applied on the javelin, F= 130 Newtons
Acceleration gained by the javelin,

Write the expression for the unbalanced force based on the newtons second law.

Here m is the mass of unbalanced forces.
Substitute the values in the above equation.

Hence, the mass of the javelin is 16.25 kg.