To find the midpoint, you're trying to bisect the line (cut it in half). The equation you'll need for this is (x1 + x2 / 2, y1 +y2 / 2). You're using the starting point of the line and the ending point of the line, and dividing it by two to get to the middle.
(3+-5/2 , -1 + -3/2)
(-2/2 , -4/2)
(-1,-2) is your midpoint