Answer:
I think that answer would be 6.
Step-by-step explanation:
If you think about what they are telling you, you will see that XY is a straight line. If the coordinates of y are given and the x is 0, then the midpoint is 3 units away. Midpoint makes the length divided equal so i would add three to the midpoint's x value to get 6. Comment if you have trouble following:)
