You would conventionally find the midpoint M of two given points by finding their average:
M = (P+Q)/2 = ((3-7)/2, (-1-6)/2) = (-2, -7/2)
You can also subtract one point from the other and add half the difference to the point that was subtracted.
 : M = P +(Q -P)/2 = (3 +(-7-3)/2, -1 +(-6-(-1))/2) = (3 -5, -1 -5/2) = (-2, -7/2)
It looks like there are some missing "divided by 2" from your answer choices.