Hello, jasonlewiz!

A circle with the center at C(5,1) has a diameter that terminates at A(6,3).

Find the other end of the diameter. Did you make a sketch? . . . The answer will be obvious.

Code:

| A
| o
| ↑
| ↑2
| C ↑
| + ← ← o → → +
| ↓ 1
--+---↓-----+----------
| ↓ 5
| o
| B

To get from the center $\displaystyle C(5,1)$ to point $\displaystyle A(6,3)$,

. . we move 1 unit right and 2 units up.

Hence, to get the other end of the diameter $\displaystyle B$,

. . we start at $\displaystyle C$ and move 1 unit *left* and 2 units *down*.

Got it?