For Part A
The kinetic energy of the particles is determined by the formula:
E=1/2mv2
Where E is the kinetic energy, m is the mass and v is the velocity of the particles.
Part A is asking for the total kinetic energy of the plate before the change, and this is equal to half the mass of the plate times its velocity squared
In this case this equals 533.1 J
For Part B
The potential energy of the plate after the change is equal to the weight of the plate times it's height above the ground