How are P and ((z1*s2 - z2*s1)/(r*(s1-s2))) calculated?
How is "P" calculated?


P is one of the parameters for the secp256k1 curve as defined in http://www.secg.org/sec2-v2.pdf.


  Which calculator/script is used to calculate z1*s2, in here ((z1*s2 - z2*s1)/(r*(s1-s2))) and so on????


Any calculator you want. They are just integers (albeit large integers) so you can use anything that can do large integer computations.