One way to find the midpoint is to start at either endpoint (A or B), and count how many spaces it takes to get to the other endpoint. You should count exactly 5 spaces. This means that the midpoint will have us travel 5/2 = 2.5 spaces when we start at either endpoint and walk to the other side.
If we start at point A and move 2.5 units up, then we'll arrive at (-2, 1.5)
If you start at point B (where the red x is) and move 2.5 units down, you'll get to (-2, 1.5)
The midpoint is located at (-2, 1.5)
------------------------------------------
Another method is to use the coordinates of A and B. Point A is located at (-2, -1) and point B is located at (-2, 4)
Focus on the x coordinates for now only. The x coordinates are -2 and -2, so they are the same in this case. Adding them up leads to -2+(-2) = -4. Then we divide by 2 to get -4/2 = -2. It turns out that if the coordinates are the same, then adding them up and dividing by two leads to the same coordinate value.
Now move onto the y coordinates. Points A and B have y coordinates of -1 and 4 respectively. They add to -1+4 = 3 and that cuts in half to 3/2 = 1.5
The results of -2 and 1.5 we got in the previous two paragraphs then immediately lead to the midpoint's exact location of (-2, 1.5)