A car leaves Sun City and takes the road towards Mars City, which is 200 km away from Sun City. The car moves at a constant speed of 70 km per hour. At the same time a truck leaves Mars City on the same road but in the opposite direction. The truck moves at a constant speed of 60 km per hour. a Find an equation for the distance from the car to Sun City, c km, in terms of the time x in hours since the car left Sun City.