Explanation:
your teacher was very friendly : he/she told you the solution in orange color inside the question itself.
all you need to do is enter the numbers of the points' coordinates and simply calculate.
what stopped you ?
so,
M = ((-5 + 1)/2, (1 + -3)/2) = (-4/2, -2/2) = (-2, -1)
it is really that simple.