input: Write a function that calculates the objective function of a linear equation represented by `ax + by = c`. The function should take three parameters `a`, `b`, and `c`, and return the value of the objective function. The objective function is defined as `a*x + b*y - c`. If the input values for `a`, `b`, and `c` are provided, the function should return the result of the objective function.
  
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  # For example:
  assert linear_eq(3, 5, 7) == 0
  assert linear_eq(2, 6, 8) == 4
output: def linear_eq(a, b, c):
      return a*x + b*y - c