Take a look at the x-coordinates of the two given points: -8 and 5. If we depart from the left endpoint (-8,8) to the right in the direction of the midpoint (5,-3), we see that x increases by 13 as we approach the midpoint. The other endpoint is twice as far (26 units instead of 13 units) from the first one. Thus, the x-coordinate of the right endpoint is 5+13, or 18.
Take similar actions withe the y-coordinates. From y = 8 at the left endpoint, we end up at y = -3 at the midpoint, a decrease in y of 11. Decrease y again by 11 from the midpoint y-coordinate, -3, ending up with -14.
The right endpoint is thus (18, -14).