Can someone me on these 3 questions with explanation

When you have a set of coordinates, the first coordinate is always the x-value, and the second is always the y-value. In the first coordinate given, (1,-2), you would go right 1 from the origin first, and then down 2. For the second coordinate, (3,2), you would go right 3 from the origin and then up 2 from the origin. One way to remember which order to go in is this: you always have to go into a building before you can go up or down the stairs. This should help you to remember to go left or right first, and then go up or down. 

17.
We have a line drawn from the coordinates (1,-2) and (3,2). Just by glancing at this line, we can see that the coordinate (2,0) is also on the line. Remember that (2,0) means that you go two spaces to the right and 0 spaces down.

18.
This question is rather awkwardly worded, but I'm guessing that by the two coordinates, it means (1,-2) and (3,2). In this case, the third vertex can be any point you want, as long as it's on the same coordinate plane and isn't on the line you drew between the two points. If you drew it on another coordinate plane, it would become a 3D figure, and if you drew it on the same line, it would still just be a line, and not a triangle.

19.
If you drew a horizontal line on a coordinate plane, all the y-values on the line would be the same. This is because, as a horizontal line, the line will never go up or down, so the y-value won't change. If you went up 5 on the y-axis and drew a line straight across from there, then no matter what point you chose on the line, the y-value would always be 5.