In order to determine the kinetic energy of the roller coaster at a height of 20m, take into account that the mechanical energy is constant.
Then, when the roller coaster is at 20m, the sum of the potential energy and the kinetic energy must be equal to 352,800J.
Find the potential energy by using the following formula:

where,
m: mass = 600kg
g: gravitational acceleration constant = 9.8m/s62
h: height = 20m
Replace the previous values into the formula for U:

Now, use the following expression for the mechanical energy:

where,
E: mechanical energy = 352,800J
U: potential energy = 117,600J
K: kinetic energy = ?
Solve the equation above for K, replace the values of E and U, and simplify:

Hence, the kinetic energy of the roller coaster at 20m is 235,200J