EventHorizon:
Eraser: yes - as if you had matches, you could move some.
Two straight edges: No.
The straight edge is just to align the matches, if we were using matches,
so it's just to draw lines in this case.
Use it to draw a circle: no. the intent is not to construct a figure.
G Threat: Yes it's a polygon but I don't know how you'd guarantee the area is integral
Y-san:
Apology was sweet, but not necessary.
Since I don't smoke or otherwise set fire to things, the matches are no loss.
And I agree, the limits should be clear.
The best puzzles, I've found, have a bit of genius in the wording.
I didn't have the needed genius on this one.
Now to the answers.
EH:
I'd love to see a sketch, cuz I got lost when you mentioned two lines of length 5.
Y-san: One word - Bingo.
Still a bit of a puzzle to word the question and require this solution.
Can't rule out others by saying convex or rectilinear.
I still think this is the only solution that you could really do with matches.
Fairly easy to eyeball vertical 3, horizontal 4, line up the 5, then dent in the corner.
Rhombus seemed qualitatively harder, to me at least.
I read the puzzle a couple days ago and liked it, even tho it didnt come with a solution.
In fact, it asked to make A=4, which I envisioned as 345 with two bottom squares missing.
So You did it one better. I had to draw it to convince myself. Nice.