Get a certain position..

I am trying to get the position that starts at position A, must pass thru position B and go more the same as the distance between
position A and B in the same angle. Looks like I need some Math, isn’t it ?

Can anyone help me please ?

Vector3 A;
Vector3 B;

Vector3 result = ( B - A ) * 2 + A ;

// Or

Vector3 result = 2 * B - A;