Math Spoiler
@critical
Top and botom are both 9-x, but there is also two overlaps, both x long, so the x cancels itself out.
Visually, you can place the 4cm and 5cm walls next to each other, creating a gap on the top the exact length of the horisontal line in the middle.
Math Spoiler
Math Spoiler
@critical
I needed a fucking paper... My brain is as smooth as ever.
Not enough info
So P=2*6 + 2*(9-x) = 12+18-2*x = 30-2*x
0<x<4
What am I missing?