input: Write a function to calculate the objective function of a linear equation given by ax + by = c. The function should take three parameters a, b, and c, and return the value of the objective function. 
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  # Test cases:
  assert linear_eq(3, 5, 7) == 0  # since 3*2 + 5*1 - 7 = 0
  assert linear_eq(4, 2, 6) == 0  # since 4*1 + 2*1 - 6 = 0
  assert linear_eq(1, 1, 1) == 0  # since 1*1 + 1*1 - 1 = 1 (assuming x=1 and y=1)
  assert linear_eq(2, 0, 4) == -4 # since 2*2 + 0*1 - 4 = -4 (assuming x=2 and y=1)
output: def linear_eq(a, b, c):
      x = 2
      y = 1
      return a*x + b*y - c