Please help with this exercise III

[Bill Spight]

@Bill:

I meant b and c, which may be called 1-3 points or 3-1 points. a is a 1-1 point.

Sorry! I did not understand "1-3" was go-speak, but thought it meant "the value of the points 1 to 3" (therefore my question: which three points)

My bad.

But now you know. Go players specify coordinates from the corners, without particular regard for which exact point they are referring to, unless it matters. Then they will use coordinates like A-17 or C-19.