input: Write a function to calculate the objective function of a linear equation in the form ax + by = c. The function should return the result of the objective function when given the values of a, b, and c. If no values for x and y are provided, the function should return the expression a*x + b*y - c.
  
  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) == -6
  assert linear_eq(1, 1, 1) == -1
  assert linear_eq(0, 0, 0) == 0
  assert linear_eq(10, -5, 2) == 3
output: def linear_eq(a, b, c):
      return a*x + b*y - c