Suppose point j has coordinate of -2 and JK=4. Then what is the possible coordinate(s) for K?

Think of the x-axis, with point J at -2.
Then, think of point K being on the x-axis also. If JK = 4, then it means the distance from J to K is 4.
If you start at -2, and go 4 to the left, you will be at -6, which is one answer for K.
If you start at -2, and go 4 to the right, you will be at +2, which is the other answer for K.
This assumes you stay on the x-axis. Otherwise, you would have a circle!