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