Hi,

For this, I suggest you take the initial velocity to be 'u'.

Then, after its speed is increased take the final velocity to be 'v' and v=u+3.

Now, because mass is constant and due to conservation of energy, the kinetic energy in both the cases will be conserved. So, you can apply this law to equate both energies and find the value of 'u'.

Give it a try and feel free to ask if you have any more queries.