an 'equable' shape has the same value for the perimeter as the value for the area
this work can involve setting up and solving linear equations and forming linear relationships
and generalisations that can be proved
an orthogonal polygon just has right angles and 270 degree angles (that I call left angles) inside it
[sometimes these are called 'rectilinear polygons'
but 'rectilinear' seems to mean 'bounded by straight lines' rather than involving multiples of 90 angles]