input: Write a function that calculates the objective function of a linear equation. This function should take three parameters: `a`, `b`, and `c`, which represent the coefficients in the equation `ax + by = c`. The function should return the value of the objective function, which is `a*x + b*y - c`.
  
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  assert linear_eq(3, 5, 7) == None
  assert linear_eq(1, 2, 3) == None
  assert linear_eq(4, 6, 10) == None
output: def linear_eq(a, b, c):
      return a*x + b*y - c