what's the endpoint if the coordinates for the endpoint and midpoint are (6,5) and (1,2)?

Answer 1

From what you have written, coordinate for the endpoint is (6,5)

Answer 2

the coordinates of B are
B = M + (M-A) = (1,2) + ((1-6),(2-5)) = (1,2)+(-5,-3) = (-4,-1)