input: Write a function to calculate the objective function of a linear equation given by ax + by = c. The function should take three parameters a, b, and c, and return the value of the objective function.
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  # Test cases
  assert linear_eq(3, 5, 7) == 3*x + 5*y - 7
  assert linear_eq(1, 2, 3) == 1*x + 2*y - 3
  assert linear_eq(0, 0, 0) == 0
output: def linear_eq(a, b, c):
      return a*x + b*y - c