input: Write a function to calculate the objective function of a linear equation of the form ax + by = c. The function should take three parameters a, b, and c and return the difference between the left-hand side and the right-hand side of the equation. Provide an example of how to use this function and assert the output.
  
  def linear_eq(a, b, c):
      return lambda x, y: a*x + b*y - c
  
  # For example:
  sol = linear_eq(3, 5, 7)
  assert sol(1, -2) == -4
  assert sol(0, 0) == -7
  assert sol(2, 1) == 0
output: def linear_eq(a, b, c):
      return lambda x, y: a*x + b*y - c