input: Write a function that calculates the objective function of a linear equation using the formula ax + by = c. The function should take three arguments: a, b, and c, and return the value resulting from the equation a*x + b*y - c. In the given formula, x and y are assumed to be predefined constants.
  
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  # Example usage:
  assert linear_eq(3, 5, 7) == 6
  assert linear_eq(1, 2, 3) == 0
  assert linear_eq(-1, -1, 2) == -1
  assert linear_eq(0, 0, 0) == 0
  assert linear_eq(2, 3, 5) == -1
output: def linear_eq(a, b, c):
      x = 2
      y = 1
      return a*x + b*y - c