a) Ok. So there are three midpoints. One on JK, one on JL, and another on KL.
You said you just need a line through the midpoint. There infinitely many that cross through a single point, but we can assume that another point on the line is at the origin for simplicity.
So the midpoint of JL is the sum of the coordinates divided by 2. So x = (2-2)/2 = 0 and y = (-2-5)/2 = -7/2. So now you have the midpoint (0, -7/2). Now you can use any other point to make a line. I'd use the origin to make it simple. So you have two points, (0,0) and (0, -7/2). That's all you need for the line.
Repeat the process for the other two midpoints.