Answer: 21 units
Step-by-step explanation: Since the y-coordinates are the same, you need to find only the difference in the x-values.
Since one is positive and one is negative, (on different sides of the y-axis) you add the distances of each coordinate from there.
Or, remember that finding the difference between two numbers is a subtraction equation, and subtracting a negative is the same as adding a positive.
15 -(-6) = x
15 + 6 = x
21 = x
