input: Write a function to calculate the objective function of a linear equation given the coefficients a, b, and c. The function should return the expression 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
output: def linear_eq(a, b, c):
      return a*x + b*y - c