Abook with a mass of 1 kg is dropped from a height of 3 m. what is the potential energy of the book when it reaches the floor?

Well, first of all, the formula for finding potential energy is;
PE=mgh
  Where; m is the mass
               g is the gravitational force or acceleration due to gravity
                h is the height. 
   Anyway, according to the question, the mass is 1kg, the acceleration due to gravity has a constant value of 10ms² . And the height is 3m. Now you just have to use all these in the formula. So;
mgh= 1 x 10 x 3. That will be 30. And the unit of potential energy is Joule. So the answer is 30 joules. Hope i helped. Have a nice day.