input: Write a function that calculates the objective function of a linear equation.
  
  def linear_eq(a, b, c, x, y):
      return a*x + b*y - c
  
  assert linear_eq(3, 5, 7, 1, 1) == 1
  assert linear_eq(2, 4, 6, 2, 0) == -2
  assert linear_eq(1, 1, 1, 0, 0) == -1
  assert linear_eq(0, 0, 0, 5, 5) == 0
  assert linear_eq(-1, -1, -1, -1, -1) == 1
output: def linear_eq(a, b, c, x, y):
      return a*x + b*y - c