To solve this problem, it's important to understand what a midpoint and endpoint are on a line. A midpoint is the point at the center of the line, and an endpoint is one of two points on a line where it terminates, or ends. Because the midpoint is at the center, or middle of the line, the distance from the midpoint to either endpoint is exactly equal. Knowing this, we can figure out the other endpoint of the line given in this problem.
Remember, the distance from the endpoint to the midpoint is going to equal to the distance between the points given. So all you really need to do here is find the difference between the x and y coordinates. You can do this by subtract the endpoint's x and y coordinate from the respective midpoint's x and y coordinate, as shown below:
For the x coordinates,
20 - 17 = 3
For the y coordinates,
21 - 19 = 2
So now we take these differences, and add them to the midpoint's respective coordinates, as shown:
For the x coordinate,
20 + 3 = 23
For the y coordinate,
21 + 2 = 23
So the other endpoint is at (23, 23). If you want an easy repeatable equation for this, it could look like the one below:
(xe2,ye2 ) = (xm + xm - xe1, ym + ym - ye1), where xe2 = endpoint you're looking for's x coordinate
ye2 = endpoint you're looking for's y coordinate
xm = midpoint x coordinate
ym = midpoint y coordinate
xe1 = given endpoint's x coordinate
ye1 = given endpoint's y coordinate