An airplane makes a round-trip supply run that takes a total of 6 hours and is 350 miles each direction. The air current going to the destination is in the direction of the plane at 20 miles per hour. The air current traveling back to the starting point is against the plane at 20 miles per hour. Let x represent the speed of the airplane, in miles per hour, when there is no wind. Replace the values of a, b and c to create the equation that describes this situation.