assume Z1 = 1 assume Z2 = 1 compute A = Y2-Y1 compute AA = A^2 compute B = X2-X1 compute CC = B^2 compute F = X1 CC compute Z3 = 2 CC compute D = X2 Z3 compute ZZ3 = Z3^2 compute X3 = 2 (AA-F)-a Z3-D compute Y3 = ((A+B)^2-AA-CC) (D-X3)-Y2 ZZ3