input: Write a function to calculate the objective function of a linear equation given by ax + by = c. The function should return the value of the objective function.
  
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  assert linear_eq(3, 5, 7) == 0
  assert linear_eq(2, 4, 6) == 0
  assert linear_eq(1, 1, -1) == 0
  assert linear_eq(-2, 3, 0) == 0
  assert linear_eq(0, 0, 0) == 0
output: def linear_eq(a, b, c):
      return a*x + b*y - c