A500.-kg roller coaster car starts from rest at the top of a 60.0-meter hill. find its potential energy when it is halfway to the bottom.

1.47x10^5 Joules  
The gravitational potential energy will be the mass of the object, multiplied by the height upon which it can drop, multiplied by the local gravitational acceleration. And since it started at the top of a 60.0 meter hill, halfway will be at 30.0 meters. So  
500 kg * 30.0 m * 9.8 m/s^2 = 147000 kg*m^2/s^ = 147000 Joules.  
Using scientific notation and 3 significant figures gives 1.47x10^5 Joules.