If you have two points (a, b) and (c, d) the midpoint is calculated as ([a + c] / 2, [b + d] / 2)

A(1,-1)

B(4,-1)

C(4,3)

So

Mid point of AB = ([1+4]/2, [-1-1]/2)

=(5/2, -2/2)

= (5/2,-1)

Mid point of BC = ([4+4]/2, [-1+3]/2)

=(8/2, 2/2)

= (4,1)

Mid point of AC = ([1+4]/2, [-1+3]/2)

=(5/2, 2/2)

= (5/2,1)

So there are three midpoints which are (5/2,-1), (4,1) and (5/2,1)

A(1,-1)

B(4,-1)

C(4,3)

So

Mid point of AB = ([1+4]/2, [-1-1]/2)

=(5/2, -2/2)

= (5/2,-1)

Mid point of BC = ([4+4]/2, [-1+3]/2)

=(8/2, 2/2)

= (4,1)

Mid point of AC = ([1+4]/2, [-1+3]/2)

=(5/2, 2/2)

= (5/2,1)

So there are three midpoints which are (5/2,-1), (4,1) and (5/2,1)