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.