Astone was thrown vertically downwards from the top of a tower. if the initial velocity was 3.0 m/s, calculate the velocity with which the stone hit the ground 2 seconds later?

You haven't specified where the experiment took place.  I'm going to
assume that it was somewhere on Earth.  That's important.

The acceleration of gravity on or near the surface of the Earth is about 9.8 m/s² . 
That means that a falling object falls 9.8 m/s faster every second.

So in 2 seconds of falling, its downward speed increases (2 x 9.8) = 19.6 m/s .

If the stone was falling at the rate of 3.0 m/s when the 2 seconds began,
then its downward speed at the end of the 2 seconds was

                   (3.0 m/s) + (19.6 m/s)  =  22.6 m/s

The modern-day generator works on the principle of electromagnetic induction discovered by michael faraday in 1831-32. faraday discovered that the above flow of electric charges could be induced by moving an electrical conductor, such as a wire that contains electric charges, in a magnetic field.