Elliptical Curve Cryptography decryption resulting point at infinity?
Summarizing the answer from the comments, starting with poncho:


  Since point $(27, 40)$ and $(27, 3)$ is a straight line, it will resulting
  point infinity.'; actually, not. $(27,40)−(27,3)=(27,40)+(27,40)$, that
  is, we have a point doubling...


which resulted in 


  how can $(27, 3)$ can turn to $(27, 40)$ and not $(27, -3)$?


which is resolved by user94293


  $(27,40)$ and $(27,−3)$ represent the same point. Computations are done in
  $\mathbb F_{43}: −3\bmod43=40$