Hello everyone, good! thanks to the help of MrFreeDragon and BrewMaster I now have a good basis for adding 2 points on an elliptical curve.

As a reminder :

If you want to double point P in order to receive R = P + P, you should make the following:
c = 3 * P.x * Px * invert (2 * P.y)% modulo
R.x = (c * c - 2 * P.x)% modulo
R.y = (c * (P.x - R.x) - P.y)% modulo
modulo = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F
and
If you want to add 2 points P and Q (2 different non-zero Points) there is another formula for R = P + Q:
dx = (Q.x - P.x)% modulo
dy = (Q.y - P.y)% modulo
c = dy * invert (dx)% modulo
R.x = (c * c - P.x - Q.x)% modulo
R.y = (c * (P.x - R.x) - P.y)% modulo

I have now no problem with this but as you can imagine, I still have 1 problem ^^, certainly due to my approximate understanding of English I am unable to find a formula for point to point subtraction. Is this also possible?

Thanks in advance (again^^)