Elliptic Curve: $y^2 = x^3 + 3x + 1 \bmod 7$
Point : $P(0,1)$
First I calculate lambda for when $P1=P2$
$\lambda = (3x_1^2 +a)/2y_1 = (0 + 3)/2(1) = 3/2$
now calculating $X_3 = \lambda^2 - x_1 - x_2 = (3/2)^2 = 9/4$
Finally Calculating $Y_3 = \lambda(x_1 - x_3) - y_1 = 3/2(0-9/4) - 1 = 19/8$
However, the expected answer is $(4,0)$ I am just wondering where I went wrong thanks in advance.