radius
Is called a radius.
yes
There are two possible answers given the information. What isn't given is if the second point is one third of the way from the known or unknown endpoint. Say the known endpoint is (xe,ye) and the point one third of the way along is (xt,yt). If the point one third of the way is closest to the known endpoint, the other endpoint would be (xe+3*(xt-xe), ye+3*(yt-ye)). This is probably the answer implied by your question. If the point is closest to the unknown endpoint the the unknown endpoint is (xe+(3/2)*(xt-xe), ye+(3/2)*(yt-ye)).
Yes
The endpoint has no dimension. It has no endpoint.
If the coordinate of A is x, and that of the midpoint of AB, M, is m then the distance AM is m-x so the distance AB = 2*(m-x) So the coordinate of B is x + 2*(m-x) = 2m-x For coordinates in more than one dimension, apply the above rule separately for each dimension.
Yes, a ray has only one endpoint.
It has two endpoints. A ray has one endpoint.
A radius.
a radius
A ray has one endpoint and continues in the other direction infinitely.
It is a ray.
diameter
radius
The other endpoint is -5,-8.
By definition a line has no endpoint as it can extend to negative and positive infinity. A ray has one endpoint and a segment has two endpoints.