input: Write a function named linear_eq that calculates the objective function of a linear equation ax + by = c. The function should take three parameters a, b, and c, and return the result of a*x + b*y - c.
  
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  # For example:
  result = linear_eq(3, 5, 7)
  print(result)
  
  assert linear_eq(3, 5, 7) == 3*x + 5*y - 7
  assert linear_eq(2, 4, 6) == 2*x + 4*y - 6
  assert linear_eq(1, 1, 1) == x + y - 1
  assert linear_eq(0, 0, 0) == 0
output: def linear_eq(a, b, c):
      return a*x + b*y - c