Answer:
(3,7) for the first line, and (12,0) for the second one.
Step-by-step explanation:
Hi Isabella,
1) The Midpoint of a line, when it comes to Analytical Geometry, is calculated as Mean of two points it follows:

2) Each segment has two endpoints, and their midpoints, namely:
a) (1,-9) and its midpoint (2,-1)
b) (-2,18) and its midpoint (5,9)
3) Calculating. You need to be careful to not sum the wrong coordinates.
So be attentive!
The first line a

So (3,7) is the other endpoint whose segment starts at (1,-9)
The second line b endpoint at (-2,18) and its midpoint (5,9)

So (12,0) it is the other endpoint.
Take a look at the graph below: