The midpoint of AB is M (-1,5). If the coordinates of A are (3,-8), what are the coordinates of B

(-5,18)

Explanation:
The midpoint, M(-1,5), should be in the middle of line AB (hence the name midpoint). So, the distance for A to M is equal to the distance from M to B. If A to M is left 4 and up 13, then M to B should be left 4 and up 13. This is (-5,18).