You may want to review distances between points first.

This also works in reverse. That is, suppose you have to find a
point (−2 ⁄ 5, *y*) at distance 1
from (−1, −1). Then you first square the distance, 1,
getting 1 again. Then you take the difference of
the *x*-coordinates, which is 3 ⁄ 5, and
square that, getting 9 ⁄ 25. Then
you *subtract* that from the square of the distance,
getting 16 ⁄ 25. Finally, you take the square
root of that, getting 3 ⁄ 5. That's the
difference between the *y*-coordinates. Now there are two
possible answers for *y*, −1 +
3 ⁄ 5 = −2 ⁄ 5 and
−1 − 3 ⁄ 5 =
−8 ⁄ 5.

Of course, it works the same if you're asked to find *x*
instead of *y*.