Finding the midpoint between two points on a line involves three steps:
1. Find the distance between the two points
2. Find half of that distance
3. Either add that half to the lowest point, or subtract it from the highest point.
To find the distance between 52 and -18, we can take the absolute value of their difference. The reason we take the absolute value is to force the sign to be positive, since distances can never be negative. -18 - 52 = -70, and |-70| = 70.
Second, we find half of that distance, which in this case would be 70/2 = 35.
Finally, we add that value to -18 to find a midpoint of -18 + 35 = 17.