input: Write a function that calculates the objective function of a linear equation in the form ax + by = c. The function should take in three parameters: a, b, and c, and 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) == a*x + b*y - 7
  assert linear_eq(2, 4, 6) == a*x + b*y - 6
  assert linear_eq(1, 1, 1) == a*x + b*y - 1
  assert linear_eq(0, 5, 0) == a*x + b*y - 0
  assert linear_eq(10, 0, 20) == a*x + b*y - 20
  
  # where x and y are variables not provided in the function parameters.
output: def linear_eq(a, b, c):
      return a*x + b*y - c