What is the answer to this 3x-15= ax+7

Im literally having a mental breakdown

Cody started biking to the park traveling 15 mph, after some time the bike got a flat so cody walked the rest of the way, traveling 4 mph. if the total trip to the park took 6 hours and it was 46 miles away, how long did cody travel at each speed? solve using rate times time= distance

What is the difference between the equations of a vertical and a horizontal line?