The distance from -21 to 18 would be 39 units ( -21 to 0 is 21 and from 0 to 18 is 18 , 21 +18 = 39)
To find the midpoint distance, divide the total distance by 2:
x = 39/2
x = 19.5
Now to find the midpoint, either add 19.5 to -21 or subtract 19.5 from 18.
Midpoint = -1.5