input: Write a function to create a Car object and return its string representation.
  assert create_car('Toyota', 'Camry', 'Blue', 4) == 'Toyota, Camry, Blue, 4'
  assert create_car('Honda', 'Civic', 'Red', 4) == 'Honda, Civic, Red, 4'
  assert create_car('Ford', 'Mustang', 'Black', 4) == 'Ford, Mustang, Black, 4'
output: def create_car(make, model, color, doors):
      return f"{make}, {model}, {color}, {doors}"